I think it would be nice if people you're fighting with could surrender, at least once they see how mighty you are and you've killed most of their friends, then—if you can stop your attack in time—you could offer them a chance to join you. But I suppose this is a feature that would have to be built into the foundation game.
I only kill people and things that attack me, but would much rather win them over.

Sometimes they put their hands on their heads, but then they run away.