I've been wanting to do one of these for a while and I finally got around to doing it. This mod keeps the Maxis PT and adds three of my own making, complete with infant faces! One of them is just a Maxis face template with larger eyes and pointy ears, and the other two are various alien sims I created for my own game, just with all the CC removed.

This theoretically should be basegame compatible, none of them use assets from any of the expansions. I also tested to the birth of the babies and you can see the babies in the last 3 screenshots, in order of the parent (the victim sims had Face 2 so you can easily spot the differences in the infant faces).

Credits: the original Multi-PT mod for instructions on how to make your own