GCode for Mach3 PMDX126 Board

Not likely, no. LightBurn doesn’t support Mach3, and normally uses spindle speed to control the laser power output, and the coolant mist / flood commands to turn on / off the laser (M03 / M05).

You could run a script to do the command substitution you need, but it’s not something that can be done from within LightBurn at this time.