I want to make my SS Shimano coaster hub to a QR version.
Does any know if it's an axle in the market who will fit directly ?

I'm sure there is. Take it down to your lbs and they should have one.

Hmmm I'm my own lbs. I don't think its the same threading. I've to go down to my workshop and measure it.

Does not sound like a good idea. The torque bar mounting to the chain stay is not quick release anyway. The forces on the wheel are different from a geared bike with rim brakes. The nuts are safer.

The torque bar mounting is a minor problem. I have to go the old way with an U-clamp on the chainstay.

I agree with that.

I'm concerned about the nuts being replaced by the quick release lever. If you are very careful and you are the only one to ever change the wheel, it could be fine, but.. if anyone else gets a hold of it...
With the torque bar on one side taking braking force there is a lot more force to twist the wheel in the dropouts. The force is very one sided. With rim brakes it's more even. With the nuts each side is secured independently. If one nut is loose, or is loosened by the wheel twisting, the other nut can still be tight. With the quick release if one moves, they both may come loose. With the coaster brake, axle nuts on each side are safer. That's my only concern.

With your theory you should newer have to use f.ex. a disc brake and qr together. I think it's the same one-side force.

You could be right. Makes sense. I just wanted put my 2c worth in because it came from a trusted source and it seemed safety related.