How to change the axis of bone of humanoid model?

I had this problem too, and after a lot of research and trai land (t)error, I have come to the conclusion that it's not possible to change the axis orientation of a bone... At least not in 3D Studio Max. Maybe your 3D modelling application has such a feature, but I highly doubt it. Anyway, there are ways around the issues that come with this limitation (namely that the bone rotation gets screwed up when using LookAt and similar rotation functions):

If a bone will be used with LookAt etc., define a rotation offset for it's eulerangles:

 public Transform torsoPivotBone;
 public Vector3 torsoBoneLookAtOffset = new Vector3(0f, -90f, -100f);

And to use LookAt:

 void LateUpdate() {
      torsoPivotBone.LookAt(aimingTargetPos);        torsoPivotBone.Rotate(torsoBoneLookAtOffset);
 }

torsoPivotBone being a bone Transform that controls the upper body, for example (like, one of the spine /chest bones).

What it does? LateUpdate() gets called after rotation and movement caused by animations take effect, so it's here where we can alter the bone's transform. So first we make it look at a target position (which by itself would, depending on the pivot orientation, result in very weird behaviour), and then apply our offset angles so it faces the right way after all.

Another way (which will not work as well with animations) is to temporarily re-parent the bone to a helper object that faces the right way and is rotated instead.