It is possible and very easy to do (figured out how to do these things for myself). However, the issue is that you will have human ears, which elves and tieflings don't have. So it would look weird. Elven faces for tieflings are much better options since they both have pointy ears so it won't look off when you have elven heads for tieflings. I would share my file, but since the mod isn't originally mine, I can't do it. Here is the tutorial if you wish on how to do those things yourself (hopefully, it is easy to follow):
Edit: Fixed a few grammar mistakes.
So I go to my Baldur's Gate 3 directory (under steamapps) and I get as far as data. There are none of the subsequent files after data. Also, I can't open most of the pak files and, the ones I can open, are in a script that I can't read at all. I've spent so much time trying to figure it out on my own.
Can someone help me? I'm trying to get the female, half-elf head 2 as an available head option for the Tiefling race.