Get the font file you plan to edit in BDF format.
Its encoding should start at 8481 (0x2121). For an example, consider that you have a BDF file like the following myfont14.bdf:
STARTFONT 2.1 COMMENT Sample Font FONT Myfont-Medium14 SIZE 14 75 75 ... STARTCHAR C101 ENCODING 8481 ... |