I've seen two GL1000s with the shocks from the GL1100. It looked like a straight swap and I don't remember seeing any adapters fitted. The air shock from the 1100 is supposed to transform the handling of the older model.
I seen a GL1000 a few years back with rear shocks from an 1100. It also had other stuff from the gl1100 fittted; fairing, rear trunk & saddlebags and wheels! Possibly more stuff as well that I can't remember now.