The Left/Right Ball Controllers can be animated (rotated and translated), it's the vertical bones located at the base of the toes. If you want to shift the nulls pivot from the heel to any part of foot, just animate the ball controller to where it needs to be so the pivot of the null is where you want it. Then switch to the null to continue animating. You could create a percentage pose that shifts the ball controller to the desired location so the null is in the correct position of the new pivot you want. That way it makes it easier to get back into the original placement. The only thing is, you won't get any auto rotation of the heel, if that is what you are looking for.