Maybe they can, have a look at these tools (specifically Umodel and conversion scripts): XNALara ( XPS ) 10.8.6 and tools - nakedpatch.ru. And for GPK file packing/unpacking: Tera tools - nakedpatch.ru.So I guess the root of my question comes down to can costumes be modified? If so, that would seem almost too easy since it could be worn over anything, and not require a template/remodel.
Note that I have only briefly looked at those tools, but they seem to be the right ones.