I've never done it, but I think that it can be done without too much hassle. I think that if you separated the hip of the G.I. Joe, place a lot of epoxy putty in the hip to hold the silver thing in place (to replace the O-Ring), and then, after it dried, glued the hip to the torso of a SW figure, that might do the trick.
Again, I've never tried it before, but that is how I would go about doing it.
By the way, welcome aboard, I look forward to seeing what you do with this custom!