Yes. Connect Garmin to PC. Load Google maps. Find the point on the map you want to make as a POI, right click it and go to the "What's here" option. Now go to the left panel of the map and you'll see a link icon. Click on that and then click on the SEND link. You should see a GPS option - click on that and select GPS brand and send. It will appear in your favourites on the GPS. This will send your POI's from Google maps to your GPS.
However, if you want to see the POI's you have on your GPS and edit them on the PC I'm not sure how to do that. I think you can do it with ExtraPOIEditor, and I think Mapsource has some very limited functionallity here.