I have never tried it, but my guess would be that you would need to copy the NPCs Race-type (so if they're an Imperial, make a copy of the Imperial Race), make your modifications to that Race (there's a tab for assigning Skins and Skeletons-- not sure if it's actually Meshes/models, or just Textures, unfortunately), then assign the NPC to that Race.
yeah, unfortunately it only allows you to change the textures to the race - not the mesh.
but you can change the skeleton.nif mesh to the NPC.
however, the original skeleton.nif links to the original femaleupper.nif, femalelower.nif ...
so I need to create a new skeleton.nif that links to the new curvy body femaleupper.nif, femalelower.nif ...
then I can assign this new skeleton.nif to the NPC.
problem is, I don't know how to link the curvy female body nifs to the new skeleton nif.