thats not a bad idea ... i cant say that i know how to do such a thing .. but as always there are options! if you cant find it MAKE IT!
software: write an app that would inperpret the keypress's and depending on what "mode" its in send different key presses
hardware: build a device that goes between the keypad and the computer that would again take the keypress's and depending on what mode it was in send a different keypress, you could also rig up an lcd screen off of the board to show what you were typin' (and put it some where more visable)
granted i dont know how to really do this .. but if this is your only option (im hoping its not ) there are those here that could do this.