Yeah not so much a problem for a custom race. as long as all you wanted to edit are the ears
you probably can't do hardly anything I suggest though.
Basically I would start by editing the human head nif via poly modelling. I would not in any way edit the eye nose mouth* area. Once I have my ears in a basic mesh, I would reUV that new geometry, to fit in with the old UV. I may redo the UV some what, as I can simply transfer the textures onto my new UV map anyway. then chop those ears onto the female head.
Then I would go and sculpt the highpoly and bake some new textures and normals. depending how far I was going... I could be retexturing the whole head/ body..
copy the rigging back onto the heads.
export
make new tri/egm/egt
* cept to pry open the lips slightly so that the conformulator would work properly transferring over the face morphs for the mouth. I would do this dead last anyway.
I have a working twi'lek head for oblivion bascially doing exactly this. its exactly the same for F3,
With custom races I recomend using throttlekittys condenced mouth nif, this frees up face slots for other bodypart add ons. which you could actually use to slip in your new ears, if they were to cover all traces of the vanilla ears....