How to modify animation rigs in unity?

I’m making a third person game and the player is allowed to pick up and throw a ball. I am trying to modify the player’s hand so that he looks like he is holding the ball.

One way to accomplish this is to create a separate layer in your animation controller that will override the base layer and then set it to have a mask so it only affects the hand that will hold the ball. Then you create an animation of the hand in the appropriate pose which you can activate separately from the other animations playing.

You could also create alternate versions of the idle and run animations that have the hand in the correct pose. One way to do this is to duplicate the animations from the fbx (click them in the fbx hierarchy and hit ctrl+d) which will create copies of the animations that aren’t read-only and can be edited.