My understanding of active mixing is that it is the same as passive mixing but with a motor to stir the filament in the mixing chamber. As such, it should not need any special support in firmware. If the motor needs to be controlled, I suggest you connect it to a spare fan output, but make sure you add a flyback diode if you are using a Duet WiFi prior to PCB revision 1.02, or an older Duet.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
