Just as an example, I drew some tennis racquets in pcb-rnd on the copper layer,
and got the string look by placing polygons where the strings have to go, selected the polygons, then issued the
command. I drew them in the copper layer in pcb-rnd, which supports arcs.
I cut them to the copy buffer and rotated.
I then moved them to the silk layer to allow export with arcs for the racquet frames to Kicad s-expression format. Kicad does not support arcs on copper as far as I am aware. If there are no arcs like the tennis racquet frames travelling to Kicad along with the cross-hatched polygon, then this is a non issue.
From pcb-rnd, you can do a "File:Save As",
then pick Kicad s-expression format from the list
then load the saved layout in pcbnew:
Obviously, if you already have a polygon in Kicad that you want to cross hatch for HPGL export or perhaps a capacitative sensor, it would be easiest to save it in a Kicad s-expression layout, load that into pcb-rnd, convert, then save as Kicad s-expression.