For ROUTES, get them into Basecamp and send them to the Nuvi. The Nuvi will convert them to "trips" which may or may not do what you want, but the new automotive units don't use "routes" anymore, only "trips".
If you are talking about TRACKS, there is no way to import them into any model Nuvi. This is just a limitation of the automotive units, they can only display the active track that is being recorded on the device.