Edit the: “<project_name>.kicad_pcb” file.
All files in KiCad are ascii & human readable (and editable) text files.
The file formats are described here:
I just tried @eelik 's suggestion in KiCad V4.0.7 and it works for me.
1). In PCBnew, draw some traces with “x”, on both sides, place a via with “v”.
2). End button to end the trace, Esc to exit from the trace draw funcion.
3). Drag a box around the trace segments -> Right Mouse Button -> Properties -> Layer: -> F.Cu -> OK.