There are programmable keypads from Genovation, with Cherry MX Black switches and relegendable keycaps and plastic key/keycap puller. Two-unit keycaps span two switches, so they are hard to depress though. The keypad is also quite large in size. PS/2 only (AFAIK), with a socket for your other keyboard. I don't know anything about the programming software.. maybe it is not compatible with later versions of Windows and I doubt that it would be compatible with USB adapters.
BTW. I have one that could sell, but I'll be keeping the keycap puller because it is my favourite.