>>536847437
Kinda. To modify them you implant them with a xenogerm, which is a collection of xenogenes. It erases any existing xenogenes that the pawn has and applies the new ones. For some races who only have xenogenes, it's essentially changing their race. But for those who have endogenes, the new genes will coexist with the existing ones unless there's a conflict.
https://rimworldwiki.com/wiki/Genetics