Regarding re-import. Such feature isn't officially supported. Although Skipper usually imports it correctly there are situations when it's not possible import it 100% correctly. And what you're describing is exactly the situation when it's not possible to decide whether to merge or add second behavior.
But to your original problem with Locale. You should be able to write any other field (also non-orm) to this property and after this Skipper should export it to your Php file. Although this feature is somewhere between ORM and non-ORM area, we have some internal hacks to allow users to use non-ORM fields in such situations.