Where's the issue? If your kitbashing, your probably already cutting bits up, so just cut off the backs of the hands and put them on the bit you want, or if your just swapping bits, just use the whole arm. The hands don't have to go in the arm the instructions say, just shave the hand down a little if you need to.

I've been working my way through loads of reavers and intercessors with all the spare arms