First off - you have already been helpful.
I tried Garmin's PI software and created a CSV file from the GPX that contained only the Long/Lat and GCxxxx. The software created a Custom option in the POIs of the i3, but was empty.
The i3 came with Mapsource 7, but I couldn't find the option you are referring to. Could the included Mapsource for i3 be dumped down?
The i3 is a great car Nav unit for the price. I use a handheld Garmin eTrex Legend for caching, but would like to use the i3 to get me close enough to park.