What happens if you simply select the "F.Mask" layer as the active layer, then place a graphic line on it? (Be sure to also make the F.Mask layer visible, so you can see the line you draw!)
Or, define a footprint (call it something like "Solder_Atch_Pad") consisting of a single SMT pad of a suitable size, then place a few instances of this footprint on your board as needed. You'll have to define the pad as being part of the GND net, but you'll be able to add thermal relief spokes to make soldering a little easier. Be careful whenever you re-import the netlist, since KiCAD likes to send "extra" footprints, and single-pad nets, to the bit-bucket.