... But as you are thinking, so am I. It's quite possible that adding another return will affect the rest of the house.
This might be best addressed by a professional. He may say that a very small duct would be OK but there are too many variables that are unknown. Is the basement heated? Is it finished? How big is it? What type of unit do you have? Is it 'properly installed' as is meaning you SHOULD have a duct in the basement?
Maybe others have more to offer. I'm sorry, I'm not an HVAC 'pro'. My best to ya and hope this helps.