It is not prohibited. There are no built-in applications for applications in WP7 applications, so you can use any other method.
In addition, if your application runs on a WP8 device, you can use Reflection to access InAP libraries
http://channel9.msdn.com/Shows/Inside+Windows+Phone/Inside-Windows-Phone-45--Adding-In-App-Purchase-as-a-light-up-feature-to-your-Windows -Phone-7-Games
Windows Phone Application Policy
2.1.2 If your application includes billing functionality in the application, but does not use the API for purchasing Microsoft applications provided by Windows Phone, the following requirement applies:
If you do not have an existing billing relationship with the user, your application may not require the user to provide payment information as part of the application’s experience to activate, unlock or expand the use of the application. If you do not have a pre-existing payment relationship with the user, payment and personal account information should be collected outside the application through a secure HTTPS website or as a hosted control in the application where the user can check the protection of URLs and HTTPS.
source share