I don't know if it is possible, but I think that if it wasn't hardcoded, it could be modified in a quest (or quest reward) file, maybe.
And that might be the unpopular opinion : I don't think the Accomplished Hero feature should be changed. I already find that it saves a lot of time; arguably the most boring phases of the game. However, starting straight off to Legendary seems overtuned to me. There's two type of people here : people who have little time to spare for games and those veterans of the game who are too good to do anything else but end-game. Endgame, by definition, should be at the end; you should work for it. Every game should be long and offer a lot of content; getting straight to the end part basically shortens the game and reduces its replayability.