The plenum is usually reduce after a line is ran off it to produce an even positive pressure through out the system. All so the return registers insure that there is a negative pressure in the room being fed. With out ether the flow from these new runs will be low. The problem can be solve by installing in line blower. They make them just to fit the 6" and 8" lines. Will have to add controls to the furnace to switch them on and off with the blower there. All so I would put some type of damper on the lines so that you can regulate the amount of air drawn from the system to the new rooms.