I don't think there's an official adapter. I've seen several on eBay that look decently built, but I don't know what their true capacities might be. It might be possible to purchase and install an AM General H1 hitch receiver, but you might have to remove your pintle hitch to install it.
The H1 hitch will install no problem. You must have the airlift bumper and the pintle must be removed. Only "modification" was having to drill the body brackets to mount the outside hitch brackets to.
It's an impressive mount that doesn't just mount to the bumper and body brackets, it also has braces that attach it to the rear lower control arm mounts.
Pic from about a year ago when I was nearly wrapped up on the install.
All parts purchased from HummerPartsGuy but you might find them at other places:
6008114 - RECEIVER, TRAILER HITCH
6007689B5 - BRACE, TRAILER HITCH (RH)
6007688B5 - BRACE, TRAILER HITCH (LH)
6007931 - BRACKET, HITCH (RH)
6007905 - BRACKET, HITCH (LH)
6018756 - BRACKET, TRAILER HITCH (RH)
6018757 - BRACKET, TRAILER HITCH (LH)
You'll have to hunt down some Grade 8 bolts, washers, and nuts for everything.