I have an M737 XT rear hub on one of my retro bikes. The cup is badly pitted on the original frehub body. 😕 The freehub is otherwise fine.

I acquired another freehub body, and the cup is fine. 🙂 But the freehub body is rough and noisy when it spins, even after adding grease. 🙁

Is it possible to transfer the cup from one freehub body to the other? If so, how do I go about it?

No idea which ones swap/intercharge.
Undo the free hub bolt and undo the race.
There are shims behind the race, make sure you don’t lock the
freehub when you screw the race back in.

IIRC there are 2 notches on the outer lip of the cup which, using the correct tool, will allow you to unscrew the cup and disassemble the freehub.
Lots of small balls will go everywhere.
You may be able to reassemble, but it’ll be a right fiddle. Take the spare one apart first so you don’t bugger up the better one.

Got the first one apart, no problem. There is some pitting on the freehub cones, though. But I can’t get the second one apart to save my life. 😕

Prob going to throw in the towel and just order one…