Like the title suggests I'm wondering if there's any sort of way to trade heroes to others in the diplomacy menu, or demand them also. I haven't seen anything but, maybe I'm missing something?

You can take a hero hostage as a part of advanced diplomatic demand, once you accumulate enough pressure over your opponent.

But does that mean they become yours, or they just are unavailable to their owner? I was under the impression it was the latter, since I've never actually done that option before. 

It's the latter

Next patch should inlude to sell heroes (to market I guess).