Depending on how often you need this and what are your other needs or limits, there are a couple of ways to do this.
First, if you are willing to get your hands dirty, you can edit the file directly. A graphic polygon looks like this in .kicad_pcb:
(gr_poly (pts (xy 207.01 130.81) (xy 199.39 133.35)
A keepout area looks like this:
(zone (net 0) (net_name "") (layer F.Cu) (tstamp 0) (hatch edge 0.508)
(connect_pads (clearance 0.508))
(keepout (tracks allowed) (vias allowed) (copperpour not_allowed))
(fill (arc_segments 32) (thermal_gap 0.508) (thermal_bridge_width 0.508))
(xy 203.2 90.17) (xy 198.12 100.33)
So you can draw a keepout area and copy the polygon points from your polygonal graphics into the keepout.
Maybe this could be done with a python action plugin, too.
In the unstable development version 5.99 it’s possible to convert shapes. This must be done with two conversions (I don’t know why there’s no direct conversion):
- Open the context menu of a polygon.
- Convert -> to Lines
- Keep the original selected without clicking again, then move it away or delete it.
- Select the lines.
- Context menu -> Convert -> to Rule Area.