Adding AC to forced air system
You want a second return in the ceiling of the top floor for cooling.  Otherwise, all the hot air will collect upstairs and you'll have a very difficult time cooling the space.  Any air return I've ever seen, that goes between floors, involved a wall being built out.  Usually it's hard to tell because it's the back side of a bathtub that looks like it's built out for the bathtub to fit, or a closet that's a bit narrower.  You may get lucky and already have space for it behind one of your walls, but that would require looking over a detailed floor plan to see.