I also would like to go from 126 to 130 on my Mavic hub. I searched briefly online and found this bikelist thread from Peter White.

There were three rear hub bodies, and 4 model
designations, of Mavic freewheel hubs.

The first (500) had a cap over the right side bearing. They cannot become
130mm.

The second (550) had the right bearing exposed. They can become 130mm, but
the dish is a bit high, sort of like Campy rears are now.

The third (501) and fourth (531) have the colored band on the left side.
Green came with an axle that could be either 130mm seven speed, or 135mm
seven speed. Grey came with an axle that could be either 126mm 7 speed, or
130mm 8 speed. The hub shells were identical except for the color of the
band, and the axles could be swapped between models.

I don't have the proper FW tool so can't say yet if I have the 500 or 550 (no colored band). And if I have the 550, I wouldn't know where to get a 130mm axle.