Good point.
I'd say that without Spirit Walking skill, the discorporated soul would be unable to get back to their bodies without the intervention of a shaman possessing a sufficiently high Spirit Walking skill to allow discorporation and reincorporation (or, indeed, a fetch); and that all it takes is spending 1 Magic Point to drag the soul into the spirit plane (the soul, of course, can resist) - or to put the spirit back into its body again afterwards.
The Spirit Walking skill is only needed to enter the required trance state; all that is needed to discorporate the soul is 1 Magic Point. Presumably 1 CA to actually do the thing, if it was a combat action, but no skill roll to do it - only to resist being discorporated.
So, presumably, reversing the process could take 1 CA / 1 MP and not require a skill roll, unless the soul wanted to resist returning to the host body - which admittedly would not be likely.