How to add a user-defined gcode to Lcnc?

Hi. Looking at the Table of Allocated G-codes 60-69 I can see that G65 is unallocated. How would I go about enabling G65 in Linuxcnc? 

I just got familiar with remapping and successfully implemented the G84.2 cycle example, and that works fine, but would prefer using G65 for renishaw's probing routines. 

Custom Linuxcnc compile/build required? or is there a 'user-defined gcode' technique similar to remapping?

Right. Ignore that. Perfectly doable with remap... if you don't forget to amend remap.py into /home/<user>/linuxcnc/configs/<machine>/python/

... learning curve pretty steep but all good fun.