xy-lpwm pulse gen to drive lathe spindle stepper / GRBL

This is a 5 dollar module with programmable (pushbuttons) rate and duty cycle suitable as a variable speed
signal to control a stepper motor driver. It also has tx/rx serial capability, with a 4-character ASCII word to set each of the (rate,duty-cycle) parameters.
My thought is to feed GRBL Gcode for X and Y (carriage and cross-slide) axes while using the pulse generator
to drive the lathe spindle. (Something like a 6600 cnc driver taking step signals from either the generator or
from a GRBL/CNC shield, with a simple switch to select which.)
For threading, the Z axis (spindle) is by nature synchronized to the X axis for each longitudinal pass. For non-
threading cuts, the independent spindle is preferable.
I would like to add a GRBL function, perhaps using an M-code, to ENABLE the spindle stepper driver,
and to send serial signals to the pulse gen. to specify RPM. For threading, I have my spindle set for 1 turn=1mm, so speed
is proportional to mm/min. Obviously, some tricks are needed to reflect acceleration times, but the basic
Gcode pass can be easily programmed.
I'm NOT capable of creating the code to add the serial command outputs to GRBL, so would appreciate some
help or advice on programming or pointing out problem areas. thanks so much!