I also use the Path Workbench of FreeCAD together with a Duet CNC. Here I use the Marlin or the GRBL postprocessor, both work quite well. You might have to delete some G-commands from the file that the Duet does not know.

I have also looked into writing a postprocessor for the Reprapfimeware myself. What are the special features of the Reprapfirmware compared to Marlin or GRBL?

The postprocerssors of Freecad can be found here:
https://github.com/FreeCAD/FreeCAD/tree/master/src/Mod/Path/PathScripts/post
Take the one from Marlin and adapt it for the Reprapfirmware. With the wonderful gcode reference from:
https://duet3d.dozuki.com/Wiki/Gcode