Inserting to new roll bones on an existing rig


I’m trying to insert two roll bones on an existing human rig, which has been imported as an fbx.
The roll bones must prevent the forearm to squash, when hand is rotated.

What I’m doing:

  1. create two new bones, which are aligned with the existing forearm bone (note that max saw the
    original bones as dummies)

  2. Link the two new bones to the forearm

  3. Constrain with ‘look at’ the new hand roll bone to the existing wrist, so that when you move
    it, the roll bone follows the hand ‘looking at it’ (the upnode is set to be the existing wrist

  4. Constrain with ‘orientation constrain’ the other roll bone, setting as orientation target
    the previous roll bone and the existing forearm bone.

The result is my roll bones flipping and rotating incorrectly.

I’ve tried everything: rotate pivots, xform the new bones, keep offset. Nothing works.
I know the bone matrices of the fbx are flipped by 90 degrees so the orientation of my bones
is not matching. I don’t want to rebuild the rig, because it has already being animated.
Can somebody help me?

Here are the screenshots