You can make songs available offline, meaning they're saved on your device. Not sure what else you could ask for, unless your problem is actually with DRM, not data usage.
I've suffered the same, as content agreements have changed, tracks have been made unavailable in playlists.
There's a setting in Spotify to see this; you can disable "hide unplayable tracks in playlists". You'll then see all the affected songs return, but greyed out.