I believe you can mod anything you want from the main campaign. You might need to press a few more buttons to get the wanted results but everything should work fine. What exactly do you want to do right now, so I could help you a bit more? If you only want to add quests simply try it, the larian tuts should provide enough information on how to do that. To start your main campaign mod I made a quick tut on how to start:
http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=548265#Post548265
