The easy way is to get a $20
JP1 flash cable and use RemoteMaster to program your Comcast.
But you can also do this manually with 8 keystrokes per button. Temporarily assign 21972 to AUX (Or TV if that is easier) and then follow the 994 instructions to
copy one button to another button. For example, to copy the MCE down arrow to the cable down arrow--Setup; 994 (2 blinks); AUX; Down; CBL; Down (2 blinks). It is probably worthwhile to try all of the AUX buttons first. There's no point in copying buttons which don't send a signal.
You may also need to disable volume punch through (993) and/or channel lock (973), if you haven't done so already.