When you subscribe to a premium channel, they should provide you with a podcast feed that will look like a URL. You'll be able to plug that into the Riffio app.

• If you haven't already, create a Collection in the Riffio app. 
• Click "Add to this collection". 
• The first tab on that new page is "Recommended" podcasts; the next tab is RSS (circled in the screenshot below). Tap the RSS tab.
• There's a field that will say "RSS Feed Link". Paste the link there (red rectangle in the screenshot below) and hit the Done button. 
• Once Riffio finds it, an icon and title for the podcast will display. Tap the + button beside it to add it to your Collection. 
• A green checkmark will display when it's been successfully added.