I presume you want to use a blower fan, or some other type of fan that is not readily available in 24V. You can use a 12V regulator to provide a +12V supply. Then connect your fan positive wire to +12V and the negative wire to the negative side of the fan output on the 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].
