I mentioned some time ago that I was working on a couple of scripts to help feed data into MusicBrainz. The first of these is actually usable now:
Once you’ve downloaded it and installed its dependency, you can call it using
./spotify2musicbrainz.py spotify:album:0QScw4gpLu1w9au1Mcf3FL (replacing the Spotify URI at the end with the one for the album you want to add).
It’s still in very early development, but hey, “release early, release often”. This is at least the “release early” part, no promises on the “often”.