Heat Pumps and Electric Heating - Return Air

Plenum sits above the Air handler & it sits on a Return Air Box.

Return Air Box is 40" total height (that is 18" above subfloor and 22" below subfloor)

I have three 10" return ducts connected to the return air box below the subfloor.

Can I install a return duct line into the return air box above the subfloor for more return air in an adjoining room? In addition, would this new return line adversely affect the original three return ducts below the subfloor? The reason I am considering this is because there is no return air vent in the living and dining rooms of the house.

Id say go ahead and put it in. If its to big or let to much air back in the unit . You can just block off some of the hole on the grill with a piece of metal is all. Dont forget that this air has to go through a air filter. WILL IT ????

This new return is going into the return air box which already has the air filter installed.