Let me start by saying you can't arbitrarily add supply or return runs onto a ducted system without first verifying if it can handle the additional loss of airflow. Adding more duct-work means the system is being asked to supply more CFM and BTUs. Each supply run is trying to push out air just like plumbing faucets in a house. When the faucets are all turned on and the commode(s) are flushed; do you increase or decrease pressure in the house? If half of the plumbing pipes are undersized how much worst does the pressure get? Your duct system pretty much works in the same fashion with some differences. You can't make that demand for more airflow on the system without first knowing if it can supply that addition load. A duct system has to be designed around the calculated airflow of the structure and the pressure created by the indoor blower motor. In addition to this the same amount of air you send out must come back. By adding that ductwork you can cause a flow issue on the supply or return side if not both. That’s how people end up with what we call hot or cold spot throughout the house. These are areas where it gets to hot in the summer or too cold in the winter. The rooms never satisfy because system airflow capacity is unbalanced. The best thing to do is to have a certified airflow balancing contractor evaluate and find the problems cause the airflow restriction and correct it. Otherwise you’ll only chase ghost putting vents and ducts here and there, and I promise you, you’ll never get your system to operate efficiently!