One option which is a little involved but is free and works well for me:
You can do this using Google Earth, a Google Earth Add-on called GPS Visualizer and a program called GPS Utility.
First at
www.gpsvisualizer.com you can download topo maps into Google Earth.
In Google Earth you can then draw a track or outline on the downloaded map (under the ADD tab). Then save the line as a KML file.
Then with gps utility (
http://www.gpsu.co.uk/ you can convert the KML file to a GPX file.
The GPX file can then imported into Backcountry Navigator.