Hi Stitch26,
It is true that you cannot save in the Expansion folder. It would also make no sense to change files there, because during the next complete pack installation the changed files would be replaced by the original pack contents. Please simply save changed registration bank files in the user drive or on a USB stick. Basically, it doesn't matter where (in which drive and folder) you save registration files. They will work from any location.
The only thing to note is that external files that are used in registrations (e.g. User Styles, User Multi Pads or MIDI files) may not be renamed or moved later (because otherwise they can no longer be found by the registrations in question).
Best regards,
Chris