Ah, I have a KC84 as well, same issue. I had to disable "numlock on boot" in my PCs BIOS settings. This at least made it so I don't have to remember to turn numlock off... which I would only remember after entering a wrong password several times.
Edit, re-read your message, I realize now you are using a separate keypad so you do want numlock to be on, obviously disabling numlock on boot isn't going to do much for you. I guess you're kinda boned since numlock is part of the KC84 firmware, is the keypad programmable maybe?