You can add custom girls in "Extra" sections to add into the town you want!
That’s not what I mean; I mean that when you come across a girl in a playthrough—I mean, one you didn't create yourself—that you really like, there should be an option to save her as a custom girl
Good idea! Perhaps it could be a reward for discovering all her liked/disliked interests (otherwise, you could just export her to discover them all).