JOSM shapefile "No Exporter found! Nothing saved."

I am using a shapefile of polygon boundaries with tags as a layer in JOSM to enable me to check if the data is in OSM.
After I have checked particular shapefile polygons, I would like to colour or mark them it in some way and save the shapefile, so that next time I do an editing session, I can readily see on the shapefile which I have completed or yet to do.
However, I find that any change I make to the shapefile and save I get the message “No Exporter found! Nothing saved.”
I have not managed how to find and install an exporter yet. Can JOSM help with this or should I convert he shapefile to kml or json or other before use to manage this.

I am able to use ‘save as’ and save the shapefile layer as an .osm layer, though the size of the saved file is considerably larger.
I also right-click this .osm layer in the layers window and select ‘discourage upload’
I am then able to add tags to distinguish the ‘done polygons’ from the rest, etc and save.
This solves my problem.