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 option since they both have pointy ears so it won't look off when you had 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):
P. S. Don't forget to backup your file before doing anything. Just in case you mess it up.
Great post on editing the file i hope i made it super easy for anyone to do it for themselves since i basically marked up everything with comments to make it easier to read encase people wanted to modify it for themselves and not just have thousands of lines of code that literally has no easy way to quick glance and see this is human this is elf etc. im most likely gonna make a fork of the file with custom edited stuff as well as just having silly options like swapping heads etc. i was messing around with some of the creatures and such as an example :')
Edit - Just added a link to your post in the main post
