adding a vent?

I have a 3b 2bath 1700sqft ranch with a finished basement. We closed 3 of the 4 vents in the basement since we never go down there, and I was hoping since we dont use those basements vents, we could add another vent upstairs in our great room (20x15 dining/living room). It is consistently 4-8 degrees warmer than the smaller areas like the bedrooms. Currently, the great room has 2 vents at one end of the room and I was hoping to add another at the other end of the room.

Since we keep the basement vents closed, would it be ok to add another vent in the great room or would that mess up the balance of the entire system?