KiCAD file formats via here: Help | KiCad
You need to look out for S-expression format.. starts on page 26.
And then try to play with fp_poly.. I don’t know if it can appear stand alone like gr_line or gr_arc or segment, or always needs to appear as part of a module, but it will be what you’re looking for.
See either one of the files attached there for it’s usage inside a module:
PS: you don’t want to use a zone for your purpose as it would need manual filling, connection with a net to work and can’t be loaded and placed into your layout at whim.