Short version:

The byte at offset 26 (decimal) from the start of the full lich jar (hex "59 02") has to be set to 1, 2, 3 or 4 to determine which of your characters owns the jar.

Long version:

Save the game over an expendable slot in case you screw up. Use mm7che to give yourself a full lich jar. Open the save file in your favorite hex editor. (Even the old MS-DOS "edit" program has a switch for editing binary files, e.g., "edit /50 save008.mm7". I'm sure you can find something better, though.)

Search for your character's name in the file. 364 bytes after the start of the name is the start of your inventory. Somewhere in your inventory is the lich jar (this is easier if you give all your other stuff, including worn items, to another character). The first byte of the lich jar is a capital "Y" (hex 59; decimal 89). This is followed by a 2 (filled smiley), eighteen 0's, a 1 (regular smiley), and some more 0's. The byte you need to change is the sixth 0 after that 1. Make sure you replace it: Don't just insert a new byte, because changing the length of your inventory will break the file. If your lich is, e.g., the third character in your lineup, change that byte to 03 (heart).

Sorry I can't give more precise instructions for beginners, as I don't know what editing programs you'll be using.