Where is the thermostat located?
If the upper level has no thermostat control, pulling air from that area will likely not achieve th desired result.
Also note that adding returns to a duct system that should already be sized correctly is something many of us will advise against.
Have a professional analyze your system, check your airflow and static pressures, and give you a pro recommendation.
It might even be possible to use a sensor on that level that will communicate with your tstat which would be advisable when adding more return on a level that does not utilize a thermostat.

Here is a link that might be useful: Tradewinds Heating & Cooling