I had a few units of Persian cavalry attacked by a few units of Nubian spearmen and some Egyption Chariots. Got the option to adopt the captain after I won that. And he eventually ended up as the faction leader too

I can't see why anyone would want to refuse to adopt a captain when they have the option. You don't have anything to lose, and a (usually pretty good) family member to gain. Unlike marriages, when you often get 50 year old useless wanna be family members trying to marry your daughter.