It really depends on who uses the enumeration. If this is just your own code, you can add additional values to it without any side effects, as ideally you should already explicitly check the expected values. It's usually pretty easy to keep track of any places that need editing.
It gets harder if an enum was released for other people to consume as part of the API; for example, if you add “MyNewValue” to the enumeration used when returning the web method, which can lead to the failure of existing clients (which have not yet been updated) when you send them this value - either through the code and not knowing what to do , or (more immediately) using the serialization layer, panic when he sees a line that he did not expect.
There is no such thing as partial enum; only class, structor interface.