In RTW I've added "adopted" sons, but the surname always changes to the family name. For example:

If I try make Marcus Aemilius an adopted son of Flavius Julius, in the game he will always be Marcus Julius, even if he's specified as Marcus Aemilius everywhere in descr_strat.txt.

I've also tried to start the game with a character married to a daughter of the faction leader, but I've never gotten it to work.