I finally put in the effort and ripped my magazine inserts Musikexpress and RollingStone. These I then put in a directory each. Via Bluesound I can address these directories directly for playing. As I understand it, this is not possible with Roon?
If you put the directories into the folder hierarchy of a Roon Watched Folder (Settings > Storage), then Roon will attempt to recognise each directory as an album. Since they are not actually albums as such, Roon will mark them as unidentified albums, and will then try and use any metadata present in the files. So you can help Roon by putting metadata (track number, title, track artist, “album” artist, etc.) into your ripped files.