I have done this on my K30 (cut down K60).
Of course I only did the F-row, as everything else is cut away.
I filed out the holes to fit the Cherry switches left over from the num-pad, and after fitting them, I used some hot glue to make them as stable as the switches with PCB is, then I soldered some wires to the switches.
Just solder the wires to the solder points of the connector for the membrane switches, measuring what row and column each switch is on, and solder together the matrix that way.
HOWEVER, IMHO, this is more work than it is worth.
picture with the F4-cap removed, showing the red loveliness underneath
