@dellfer a possible way around this would be to define it as an extruder. with a very large number of steps/mm and then create a tool with both extruder drivers. That way when its extruding the stepper driven pump will turn as well. Some issues with this is the cooling will be related to the extrusion feed rate. and retractions will make it turn the opposite way!
I am moving this to the firmware wish list, but I don't think using steppers as fan motors will be a high priority to implement