HG sprockets can be easily adapted to fit on a UG freehub. Only one tab, the largest one, on a HG sprocket needs to be ground down to the size of the other tabs and the HG slips right on. A dremel can do this work in one to two minutes.

Also, spacers from a HG cassette can be used on a UG freehub. This makes it possible to keep your UG rear wheel running for many decades. This is especially true for some the original 6 speed UG freehubs, which cannot be upgraded to a HG sprocket sprocket carrier.

I've converted one of these early 6 speed freehub bodies into a 7 speed HG/UG hybrid. Using 9 speed spacers and a 7 speed UG threaded 13T first sprocket, then the 2nd, 3rd, 4th, 5th, 6th and 7th sprockets were all HG. I'm using old Suntour Barends for friction shifting. You can see it in the picture below.



The beauty of using modified HG sprockets on your UG freehubs is that they are cheep and plentiful in the form of discarded HG cassettes. Obviously you need to use the lower end ones which were either held together by screws or long rivets. This allows for a great degree of customization of your gearing.

Best of luck!