If I wanted to add rooms or such. I remember it was possible back in warcraft 3 editor to do this. Hide an aspect of the map from the minimap so that the rooms did not appear on the minimap and could not be scrolled to in the game without a camera being placed there.
That's also possible in SC2. Just search for:"Set Camera Bounds" in the Trigger Editor. You should be able to make the minimap only show the part the Camera can go. You can make Regions to make specific Camera Bounds for Rooms etc =]