Attaching the Maxis hands is a bit easier than attaching Tenshii's hands, I think. Easiest is attaching them to bare arms: Just delete the hands around the forearm (not the wrist) and reattach a bare Maxis-handed arm, a lot like a shoe swap. Just snap the vertices to each other and match the bone assignments.

For outfits with long sleeves I had to add extra vertices to the wrist (twice as much, in fact), so you'll have to snap them together and merge them again before reattaching the Maxis hands. From there it's the same thing as shoe-swapping. :)