There is a previous idea that was replied to incorrectly by Spotify - Idea 137915. The point of this request and mine is to support the ability to add podcasts by URL when a podcast is a premium (paid for) subscription.
Like the original poster, I too have a podcasts that I pay to access without commercials and with extra content. Other applications allow users to add the url of the subscription directly to the podcast list.
It would be very useful to have all of my podcasts and music in one app.