There has been another discussion of this , what you might want to consider is using the spindle speed as the extruder control. Would give you a way to start and stop the extrusion, as well as control the extrusion rate. Might require that you do some translation of output codes, but that should be a faily straight-forward text substitution which could be done using find/replace all, or by writing a small program in basic/C/other language of choice.