Evan, I think you must mean your light switch is an SS13A NOT a TM751. A TM751 is a Transceiver , it has an antenna on it which picks up the RF waves from an X10 remote control.

The switch your describing, and reprogramming the bottom dim/bright button I don't think is possible. You could use one of these Leviton Transmitter Base with a four button keypad like this 4 Button Keypad providing you have wiring to the junction box or if you even have a junction box since you originally chose a wireless swith like the ss13A.  If you can wire in a switch like the one i suggested, you will still need to use PC control or a controller that can handle triggers and events.  IE:  when the bottom button of the keypad is pressed, that triggers an event which would turn all of the top 3 buttons on or off or whatever.

Hope that helps, there are other options of hardware to use, my suggestions was just one example.  I didnt look at loads too closely, but u should.