I'm not sure there's a maximum but you can definitely add to it. You have to place it in your Documents/DDSPF/ folder, though, and will only effect new leagues and not current ones.
it will load the file every time it needs to create a player (if the file isnt loaded already, i.e. if you havent created a player in the same session). So changes should appear even in existing games after a season or two as it generates not draftclasses.