Well, Game Maker can write to file, so it should be able to edit XML files if they're just stored as text files. I'd just have to learn the syntax of XML. Though out of curiosity, what particular things do you want to be moddable? For instance, changing portraits was mentioned, but what else? With the level editor you can change the music, background image, path shape, and other various things. I've never modded a game so I'm not really sure what is involved.