Short answer: I don't. I have an Air Terminal family for an Open-End Duct that I put at the Return intake in the plenum. I then manually type in an air flow value into the Air Terminal in order to put Return Air into the system. You want to positively pressurize your building anyway, so you want this value to be about 5% less than the total supply air. Initially, I'll set the Return Air flow based on my block loads, and once the Supply system is complete I'll adjust if necessary.
I understand that there's a desire to have the return air total up all the way back to the unit in the same way that Supply Air does, but the two are different animals with a plenum return system. There's no way to balance each room's return air in "real life", so what's the point in modeling it?
