That's a great idea, unfortunately current version of MS Street & Trips does this already, sort of (see below).
It will allow you to import addresses from outlook, xls, access, and a few others.
Although going through the import process it does not allow you to select the contacts you want to import. It automatically selects all of them, so you would have to import them and manually remove the ones that aren't needed.
Now depending on the GPS make/model you may need to save the street & trips as a .est file and import it into another program that talks to the GPS, because I don't think that Streets & Trips allows you to directly upload it to many GPS models.
Another option is to export your contact list to an .xls format, edit what you don't want imported, import the .xls file into Streets & Trips or another GPS utility.
I use a program called GPS Utility to upload my customer info into the GPS (Garmin Nuvi 770) this allows me to create routes with the contact info.
You can download a 60 day trial of Streets & Trips to test.
Hope this helps, let me know if you need anything, I've been down this road
