Importing OSM to Basecamp(MacOS) without having a device that displays maps

I recently bought a Fenix 3. I want to create routes in Basecamp that I can upload to the Fenix for walking/hiking and possibly running. I've downloaded a European OSM that I'd like to use in Basecamp to create routes. My problem is that I cannot work out how to load the OSM map in Basecamp. I have a .gmap folder with the map in it in my documents folder however I cannot see any functionality in Basecamp to load it. I've looked at MapManager and MapInstaller but they seem to be designed for maps that have been installed already or for Installing to a device. I know the Fenix 3 is not designed to hold maps and don't want to upload the map to the device, I just want to use it to create routes in Basecamp.