If I want to create an animation for the upper body only, so that it can be combined with standing and walking, how do I go about that? I understand the bone priority part, but not how the animation is defined to the game to get it used in that fashion. The idle animations fill-in doesn't seem to provide the right choices.

E.g. If I give an NPC a yoke to carry a couple of buckets, I'd like them to be able to stand, walk, and talk while holding the ropes at the sides. Or carry a single basket with their arms wrapped around it. The upper body idle would be needed as long as the NPC has the yoke or basket.