not sure about what they did with pathpilot but with linuxcnc I use "` through 9" on the keyboard for FRO
` = 0%
9 = 90%
0 = 100%
Try that it might work for you.
Otherwise, you can try to use adaptive feed control with m52
you can read a little about it here: http://linuxcnc.org/docs/html/gcode/...e_feed_control
you do need to connect the hal pin motion.adaptive-feed to a suitable input.
I'm not sure if your hardware setup has an analog input, that would be an easy way, otherwise you need to figure out a digital input and how to interface that to your override knob.
Definitely not a plug and play thing but definitely doable.