You're on the right track!
You want the to keep the helmet's skinning and apply
itp_attach_armature
to the item, but then have to do something a little weird.
You assign the helmet's mesh as a
submesh. The engine only deforms the primary mesh to match facekeys, any submeshes are skipped.
In the referenced image, the helmet's mesh is actually the submesh
prehawk_helmet.1
with an empty "mesh" as
prehawk_helmet
. The new primary mesh doesn't have to be empty, but know whatever is there will be deformed by the game engine.
This trick only works on the Native Engine, WSE2's updated engine modified the way helmets are deformed by facekeys so all submeshes are deformed as well.
I created the empty by using the Compute LODs tool an already decimated LOD until there were no more triangles.