With a Pro subscription, you can add music to your mashes from our licensed tracks or your own iTunes library. Songs from Spotify, Apple Music or other streaming services can't be used because of Digital Rights Management (DRM) restrictions.
Every track on Spotify and Apple Music is protected by DRM. Even paid users (Premium and Family) can’t use downloaded albums and playlists outside of these apps. This is because paying for a streaming service doesn't give you the right to use the tracks other than listening to them.
You'll need to purchase the tracks from the iTunes Store to own the tracks and use them on your 1SE projects.
To add songs you own, see How to Add Your Music.