Waypoints to POIs

It would seem possible...

...If you can get the waypoints from the handheld in to Mapsource, you could then manage/edit them then save as a gpx file.

I have a Magellan Meridian Gold that I've been usung for years & collecting my work/travel destination waypoints (POI's). The Meridian has the ability to save these to an SD card. Then I used GPSBabel to convert them to CSV format. Then I created the POI file with POI loader. The Meridian had a name size limit. I think it's 8.3.
If your handheld can download via a serial connection, you could probably do a similar procedure.
It seems that GPSBabel can convert to/from/between any GPS file format.
I'm gradually editing the names in my CSV file. I just can't bring myself to edit 400+ points in one sitting.

I also suggest using MapSource if you have it. Once you save gpx file, you have 2 options. You can copy it to Nuvi gpx folder and POI's will show in Favorites. The other option is to load gpx file using POI Loader and they will show up as Custom POI's.