I don't think you can. The 770 can only lift about 800 pounds at 24" behind the link . My 790 could barely lift the old 506 JD bush hog off the ground, I couldn't imagine lifting a round bale with it.
How big are the round bales? I bale 4 x 4's and they're supposed to be around 600 lbs., depending on moisture content. Sometimes I make them a little smaller so folks can roll them by hand.