Stepup does not support anything near this usecase 
The “only” thing comparable to this usecase can be done via the dxf import. (It can be used to get a board outline from a freecad sketch into a kicad pcb file.) The missing tool is to convert such dxf imported line segments into a polygon such that it can be used for creating complex pads.
Stepup does not even support the new pad types for the footprint -> freecad direction. Stepup has no support for the other way round at all.
Update (May 2018): stepup now supports exporting footprints (even polygon pads are supported Kicad StepUp: The Sketcher for Footprint generation)