You can select route in planner, you need to switch mode to "viewing mode" first. On desktop you can use "M" shortcut to switch modes. Unfortunately, it's not easy to add whole selected route to planned route. You have to build it on your own but "touring bike profile" will try to follow official cycling routes.
I can imagine that it is not easy, because there are also many routes that run in circles, etc. I came up with it because I looked at the Alpe Adria Route with description and stages etc. and thought to myself: Great, I'll save the route. In the description there was also a clear start and destination. In this case it would be great to have the route directly available as a planned route. But presumably it needs a manual "maintainer" that creates such routes and then publishes them. Maybe you could somehow link to such public routes. But just brainstorming. At the moment everyone must build his own Alpe Adria Route.
Routes that we are maintaining, available at https://veloplanner.com/routes, can be open in route planner.