Can I add four new ducts to my HVAC trunk line?
I admire the drawing of your existing duct-work. Using a .1" external static resistance for the duct-work and guessing that you have a 3 ton A/C system that requires 1200 CFM of air flow, I would estimate  that the 8"X20" duct is almost fully utilized as is the 8"X8" duct. The 8"X14" duct could handle all the airflow required for the basement heating. You did say that you wanted to utilize the 8"X20" duct for the basement heating. You can do it but it may unbalance the existing airflow. You may have to increase the blower speed to achieve the required air flow in that duct and add dampers to the other 2 ducts to help control the increased air flow. Adding 4 heating registers would require approximately 320 CFM more air flow you may or may not have available. I would add the ducts you require and balance the resulting system as best I could and live with the results. If the blower that is installed in your present heating system is sized for more air flow and a higher static head than I allowed for say you could easily add the 4 registers you will need. If on the other hand your blower is maxed out the system will still heat but the 2nd. floor heating may suffer. This is the best I can do since I am not at you residence and I am only guessing as to the existing set-up. good luck with your project.  I think that you will be OK. I compared your home to mine 2000 sq. ft rqnch and finished basement. Mine heats great.