Every model in game has UV maps, which set texture to model. You'll either have to change undead textures so they will fit on human models (like you already said, you will have to change faces, the rest of model should be perfectly fine), or change human model you have used in 3D editor and remake UV maps of face. For second option, check tutorials corresponding to 3D and character editing in general in modelling section. Btw, don't remake original human female model, make a copy of it to avoid ruining humans, but that should be obvious.