To answer my own question....
Go to
http://blog.sallarp.com/geojson-google-maps-editor.html
Click the icon at the top of the map that looks like a polygon
Click points to make a polygon around the area you want to select
A geojson file will be created in the window at the bottom of the screen
Copy and paste that into a text editor
Save as "
whatever.geojson" - the suffix is important
Upload the file into iTunes where requested