I believe that your initial post has been misunderstood, or at least I'm misunderstanding.
From what I can tell, you basically wish for those who are administrators to have the ability to add new forums (under a category) but restricted to whatever categories you stipulate (i.e. the ones that they are responsible for). As such, you are basically looking for a modification for Administrator permissions. I do not believe that there is anything at present to allow you to do this, but I may well be wrong.
It might be easier somehow to allow this based on "Forum Moderator" status of a category - but that again is just an assumption on my part and it may be just as easy doing it through other means. May wish to wait for somebody with greater coding knowledge to assist you somehow
Kind Regards
JayJ