I have some additional information on this. It is useful if you have users you want to save. For some reason, my Admin user wasn't defined at offset 0x80. Instead, it was at 0x104. When I followed the instructions above, it would have overwrote a different user defined at that location. I WAS ABLE TO SEE THIS BECAUSE MY HEX EDITOR ALSO DISPLAYED AN ASCII VERSION ON THE SIDE! So I could see where the user name definition began in the file.
MAKE A BACKUP OF THE UM.DAT FILE FIRST!!!!
I use VSS 6.0 on NT.
It looks like user definitions begin every 64 bytes, like at 0x80, 0xC0, 0x100, 0x140. You can see that users start with the bytes 55 55 XX XX - where XX XX seem to vary.
The only thing I did different from Finn's directions above was to start at the offset for my Admin user. And then I FOLLOWED HIS DIRECTIONS EXACTLY! It worked fine.
Of course, it is entirely possible that this was a fluke and these directions may completely screw up your dat file. SO MAKE A BACKUP OF THE FILE FIRST!!!!