keyboard mapping

Couldn't find anything related on this forum. Not sure if this would be something simple to do or not. For those who knows .. maybe, not me. What I'd like to do is assign a function to a keyboard button that is not in shortcut list. For example I'd like my Z-axis move up with a given speed by pressing one (free, not used) button. Running machine with stepper motors and Gecko drive. Any help, suggestion, anything would be appreciated. thanks

GUI is Axis 2.7.14. Yes that would be easy to do "page-up" but my question was not complete. I need extremely slow speed going down and I can manage 0.005mm no problem. And I need fast speed going up. Now that I've double checked keyboard shortcuts I see that if I previously set increment to 0.005mm and jog speed to (say) 500 mm/min then I can achieve slow increment down by pressing I+PgDwn and fast up by pressing C+PgUp.
I wish I have time (and maybe energy) to get into Pyton but my son in law is Pyton expert ... so maybe .
This kind of control is needed for EDM machine (hobby one by B. Fleming). Vertical movement in that design is by a small DC motor with high reduction. But since I have good hobby CNC machine it was better for me to use what I have.
Bit of a crude method using an old keyboard, USB interface and figuring out key matrix and few relays (four in this case) to simulate key strokes.
Output of EDM machine for motor control is 12VDC with reversing to change direction. No problem-just add diodes for up and down and I think it will work.
Thanks!

what button can use, select linuxcnc 2.8.4 debian version, select X or Y or Z axis, and then select use continues or 1mm or 0.1mm or 0,5mm need this because have lot easy use on table remote keyboard wireles mini, than go ewerytime use display and mouse select how many step move. 1mm or 0,1mm or0,01mm and 0,5mm or 5mm or etc. when search wood piece zero corners. manual. i not can add automatic G-code 38,2 system alltime come only errors lot.