problem with return air

[icedr024]

I have a bryant 3ton split unit. the house is approx 3000sq ft.Was originally there when i bought house. Have T stat on first floor along with 1 large return air grill. 2nd floor has Large return in hallway. It seems that there are no returns in any of the 4 beadrooms. As long as the doors are open return heat escapes but the rooms get nothing like first floor. I installed regular grills over the doors so that some heat can escape. Im wondering if I install grills in ceiling and run flex duct in attic if I tap into the main returns will there be that much of a difference. I really Dont want to put a second unit into the attic if i can help it. Again.. Im not a DIY guy I work on commericial A/C units.
I just feel that the rooms are being pressurized rather than air convection.

[Todd S 2]

Depending on how it was done, two returns for the system doesn't sound like enough.
Assuming there is enough return air, the transfer grilles over the doors should be enough for the rooms. That said, your idea to put one in the ceiling and tap into the return might be okay.
Again, it depends on the existing duct size. Normally, it's not too generous to begin with.