I did this with Vlad. Bring up the cheat box before you go into CAS and type in cas.fulleditmode. Then go into CAS to edit your sim. What I do is to create a child with the sim whose DNA I want (so, in my case, Vlad) and an unknown parent. Make the child a teen so you can see what they'll look like. Once you have someone whose look you like, then you can change the relationship so that the new sim is the parent and your sim is the daughter or son. Then your sim will not only have parents, but they'll have parents who share their DNA.
