But as Cigarski says, it's easy to select any roads you want on Mapsource or Basecamp by using the "Select" tool on top and then simply "pulling" the route line onto the road you actually want to use.
As Cigarski and Sea Level said, this is about the easiest way to do it. Ironically, Harley-Davidson has a great
free site to use. A few have had problems with it but I haven't. It can export to a .gpx file that can be loaded into a high-end GPS. It works great with my Garmin Nuvi 765T. You will need to make an account before it will let you save, then export your routes.
http://www.harley-davidson.com/wcm/Content/Pages/Ride_Planner/Ride_Planner.jsp?locale=en_US