The BDF source files are attached to the above mentioned U8g2 issue. You can import the bdf file into Fony.exe (fony) modify the font, export again as bdf and use bdfconv.exe from the u8g2 project to generate the font array.
One more point: You do not need to put the array into u8g2_fonts.c and update u8g2.h. You can just paste the array into your .ino file.
Oliver