As mentioned on the official JHipster website, the best way to add new fields / relationships to the JHipster user by default is to create a new object and associate it with the user with a one-to-one relationship. You can handle more relationships in a new entity. Let me name this new entity UserExtra. 
The UML diagram above shows the composition between the user and UserExtra, meaning that the user has UserExtra, which is highly dependent on the user and cannot exist without the user. In other words, there must first be a user so that we can assign a phone number.
Step 1: Create a New Object
I suggest creating a new entity using terminal: jhipster entity userExtra
or
JDL:
Step 2: change the new object
You can find the new entity class in the domain folder: 
We can match the UserExtra ID with the user ID so that we can use the UserExtra ID as a foreign key. If so, then we should no longer use the @GeneratedValue annotation for the identifier in UserExtra. Here is an example of the UserExtra class. Here is an example of a modified UserExtra:
package org.jhipster.domain; import org.hibernate.annotations.Cache; import org.hibernate.annotations.CacheConcurrencyStrategy; import javax.persistence.*; import java.io.Serializable; import java.util.Objects; /** * A UserExtra. */ @Entity @Table(name = "user_extra") @Cache(usage = CacheConcurrencyStrategy.NONSTRICT_READ_WRITE) public class UserExtra implements Serializable { private static final long serialVersionUID = 1L; @Id private Long id; @OneToOne @MapsId private User user; @Column(name = "phone") private String phone; // jhipster-needle-entity-add-field - JHipster will add fields here, do not remove public Long getId() { return id; } public void setId(Long id) { this.id = id; } public String getPhone() { return phone; } public UserExtra phone(String phone) { this.phone = phone; return this; } public void setPhone(String phone) { this.phone = phone; } public User getUser() { return user; } public UserExtra user(User user) { this.user = user; return this; } public void setUser(User user) { this.user = user; } // jhipster-needle-entity-add-getters-setters - JHipster will add getters and setters here, do not remove @Override public boolean equals(Object o) { if (this == o) { return true; } if (o == null || getClass() != o.getClass()) { return false; } UserExtra userExtra = (UserExtra) o; if (userExtra.getId() == null || getId() == null) { return false; } return Objects.equals(getId(), userExtra.getId()); } @Override public int hashCode() { return Objects.hashCode(getId()); } @Override public String toString() { return "UserExtra{" + "id=" + getId() + ", phone='" + getPhone() + "'" + "}"; } }
Step 3: Change the configuration of the new object
[You can skip this step if you used JDL in the first step]
There is a json file storing the configuration of the object: 
We need to change the configuration. Here is an example of a modified json configuration file:
{ "fluentMethods": true, "relationships": [ { "relationshipType": "one-to-one", "relationshipName": "user", "otherEntityName": "user", "otherEntityField": "id", "ownerSide": true, "otherEntityRelationshipName": "userExtra" } ], "fields": [ { "fieldName": "phone", "fieldType": "String" } ], "changelogDate": "20180317190851", "dto": "no", "service": "no", "entityTableName": "user_extra", "jpaMetamodelFiltering": false, "pagination": "no" }
There are also some XML change logs: Here are examples of change logs:
added_entity_UserExtra:
<?xml version="1.0" encoding="utf-8"?> <databaseChangeLog xmlns="http://www.liquibase.org/xml/ns/dbchangelog" xmlns:ext="http://www.liquibase.org/xml/ns/dbchangelog-ext" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-3.5.xsd http://www.liquibase.org/xml/ns/dbchangelog-ext http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-ext.xsd"> <property name="now" value="now()" dbms="h2"/> <property name="now" value="now()" dbms="mysql"/> <property name="floatType" value="float4" dbms="postgresql, h2"/> <property name="floatType" value="float" dbms="mysql, oracle, mssql"/> <changeSet id="20180317190851-1" author="jhipster"> <createTable tableName="user_extra"> <column name="user_id" type="bigint"> <constraints primaryKey="true" nullable="false"/> </column> <column name="phone" type="varchar(255)"> <constraints nullable="true" /> </column> </createTable> </changeSet> </databaseChangeLog>
added_entity_constraints_UserExtra:
<?xml version="1.0" encoding="utf-8"?> <databaseChangeLog xmlns="http://www.liquibase.org/xml/ns/dbchangelog" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-3.4.xsd"> <changeSet id="20180317190851-2" author="jhipster"> <addForeignKeyConstraint baseColumnNames="user_id" baseTableName="user_extra" constraintName="fk_user_extra_user_id" referencedColumnNames="id" referencedTableName="jhi_user"/> </changeSet> </databaseChangeLog>
Just remember that JHipster uses a table named "JHI_USER" for the "User" object and created a table called "USER_EXTRA" for the new object. We are going to name the primary key / identifier field of the new object "USER_ID":
Step 4: Modify ManagedUserVM.java
The ManagedUserVM class is a kind of DTO that extends the UserDTO that is used in the REST controller.
The purpose of this modification is to add new fields that we are going to associate with the User as additional information. Here is a sample code for this class. The phone attribute has been added to the modified class:
package org.jhipster.web.rest.vm; import org.jhipster.service.dto.UserDTO; import javax.validation.constraints.Size; public class ManagedUserVM extends UserDTO { public static final int PASSWORD_MIN_LENGTH = 4; public static final int PASSWORD_MAX_LENGTH = 100; @Size(min = PASSWORD_MIN_LENGTH, max = PASSWORD_MAX_LENGTH) private String password; private String phone; public ManagedUserVM() {
Step 5: Modify UserService.java
There is a method in this call called "registerUser" that is responsible for registering a new user:
The purpose of changing this class of service is to make it also add a UserExtra object when a user is added. Here is an example of a modified registeruser method. You can find the added code with the inscription "Create and save UserExtra entity" at the top of the comment:
public User registerUser(UserDTO userDTO, String password, String phone) { User newUser = new User(); Authority authority = authorityRepository.findOne(AuthoritiesConstants.USER); Set<Authority> authorities = new HashSet<>(); String encryptedPassword = passwordEncoder.encode(password); newUser.setLogin(userDTO.getLogin()); // new user gets initially a generated password newUser.setPassword(encryptedPassword); newUser.setFirstName(userDTO.getFirstName()); newUser.setLastName(userDTO.getLastName()); newUser.setEmail(userDTO.getEmail()); newUser.setImageUrl(userDTO.getImageUrl()); newUser.setLangKey(userDTO.getLangKey()); // new user is not active newUser.setActivated(false); // new user gets registration key newUser.setActivationKey(RandomUtil.generateActivationKey()); authorities.add(authority); newUser.setAuthorities(authorities); userRepository.save(newUser); cacheManager.getCache(UserRepository.USERS_BY_LOGIN_CACHE).evict(newUser.getLogin()); cacheManager.getCache(UserRepository.USERS_BY_EMAIL_CACHE).evict(newUser.getEmail()); log.debug("Created Information for User: {}", newUser); // Create and save the UserExtra entity UserExtra newUserExtra = new UserExtra(); newUserExtra.setUser(newUser); newUserExtra.setPhone(phone); userExtraRepository.save(newUserExtra); log.debug("Created Information for UserExtra: {}", newUserExtra); return newUser; }
Step 6: Modify AccountResource.java
This class is the REST controller responsible for the actions associated with the Account:
This controller has a method called "registerAccount" that calls the "registerUser" method of the UserService class:
User user = userService.registerUser(managedUserVM, managedUserVM.getPassword());
We need to change this line so that we can pass the new fields to the method:
@PostMapping("/register") @Timed @ResponseStatus(HttpStatus.CREATED) public void registerAccount(@Valid @RequestBody ManagedUserVM managedUserVM) { if (!checkPasswordLength(managedUserVM.getPassword())) { throw new InvalidPasswordException(); } userRepository.findOneByLogin(managedUserVM.getLogin().toLowerCase()).ifPresent(u -> {throw new LoginAlreadyUsedException();}); userRepository.findOneByEmailIgnoreCase(managedUserVM.getEmail()).ifPresent(u -> {throw new EmailAlreadyUsedException();}); User user = userService.registerUser(managedUserVM, managedUserVM.getPassword(), managedUserVM.getPhone()); mailService.sendActivationEmail(user); }
Step 7. Change the user interface.
Finally, you need to add the input element to the HTML file:
<div class="form-group"> <label class="form-control-label" for="phone" data-translate="global.form.phone">Phone number</label> <input type="tel" class="form-control" id="phone" name="phone" #phone="ngModel" placeholder="{{'global.form.phone.placeholder' | translate}}" [(ngModel)]="registerAccount.phone"> </div>
Then here is what you will have on the registration page:
And here is the data layer:
JHI_USER
USER_EXTRA
PS
The easiest way to do this is to simply add a UserExtra object with a one-to-one relationship to the user via JDL and add "userExtra" when creating a new user.