Hello, I would like to get an answer with dilemma I currently have.

We have a single unit outside with the thermostat and main return vent downstairs with no returns upstairs where all the rooms are. Two of my rooms, which are above the garage will never get enough cold or hot air inside of them. So I was thinking of installing an additional air return vents above the each door of the rooms in order to get the circulation of the air better, since the air which is coming from the unit doesn't go anywhere since there is no returns. Please tell me if it is a good idea!

Thank you and looking forward to your replies!