User blog comment:DirgeOfCerberus111/Canon Expansions, Feasible?/@comment-4608429-20131006132318

I think it's fine, as long as the fanon character doesn't become a focal point for the cannon character (though being the other way round doesn't matter). The only thing you really need to watch out for is making sure that their not somewere else (or haven't even been born yet) at the point your character interacts with them, i.e. you can't have your deamon prince of deathly doom fight the Ultramarines on his deamon world when their supposed to be stopping Hive Fleet Kraken from noming their homeworld.