Adding custom PWM output to extruder

I would like to add a preset to an extruder such as in Repetier's color mixing and virtual extruders definitions, but with the pwm output instructions going to a set of auxiliary pwm pins. Would anyone have any advice on how I might edit pins to create that output? So far I am putting pieces together to try and cobble that, either by adding a custom MCode entry or editing the output of M163 such as is used by the Repetier virtual extruder.

Where would you like to put the PWM signal? On the enable or step input?
That would not work, since a PWM signal only changes the lengths of a high signal compared to the length of the pause. Also you'd get problems with different print speeds because the frequency of PWM is fixed ( or at least not changed easily )

I believe that I can apply a controller to the PWM output so as to keep in speed and timing with the printing speed, but I see what you are saying, where I might just want to drive a pulse per time value anyways.

I was going to use pins other than the typical motor extruder pins, I had mentioned Auxilliary pwm, and might think about other data.

FWIW I am trying to drive a small metering pump to drive a paste/liquid while maintaining the traditional motors for regular plastics extrusion.

If you want to PWM only two extruders, you could use one PWM pin to control both enable pins. You just have to invert the signal for one of the extruders. Then you can set the mixing ratio between 0-100% in 255 steps.