I think it will be really useful to have an import option in the app as well as in the website.
An example case is. You are on holidays in some country you don't know. And you want to do a hiking. There are plenty resources out there on internet that allows you to download a .gpx(for example) file. But how I download it to my garmin device without a computer? It would be very straight forward if I could import it in the app and then sync to my device. In minutes I will be ready to go.