Objective: Allow users to "combine" all the categories they want to see into ONE cate

[nothing4me]

Ok. I have about 7 categories, with 5 sub-categories for each.
Objective: Allow users to "combine" all the categories they want to see into ONE category!

I would like to have a single forum that "dumps" or "shows" all the threads of those categories and posts.

So before:
MAIN CATEGORY1
-sub1
-sub2
-sub3
-sub4
-sub5

to:
MAIN CATEGORY2 with MAIN CATEGORY1 and all of it's subs.

This is like running a "Search", but I would like for it to run natively, you know? This will allow users to "combine" all the categories they want to see into ONE category!

Is there anything like it? Or can anyone make one?