By default the game engine can only handle complete models. It's also possible to force the engine to use separate models for body and head, but that requires some doing. Lots of doing, actually.
The best advice I can give is to wait for the next version of my mod
, because I'm working on just that. Here's the teaser
The second best advice would be to add this line to j00_een_default.nss:
SetAppearance(GetPlayer(), body, head);
(You will have to replace "body" with the row from appearance.2da and "head" with the row from heads.2da.)
Then compile this script with nwnnsscomp.exe. The link to the compiler is in this thread, I believe. You could also download the source code
of my mod. It includes the compiler.
This is a simplistic solution and it will break occasionally. When you transform e.g. the head will not be restored when you transform back, but j00_een_default gets called whenever you load a savegame or when a different area is loaded - in other words: a lot.