Well, actually it’s already possible to add metadata to the roon data set for unidentified albums: musicbrainz.org; one might want to argue it’s not a proper tool - especially for classical albums the process using the musicbrainz site directly is a bit tedious or so I think. I believe there was a discussion about a tagging tool which helps, but anyway: it works. 
There’s no clear statement about how roon actually imports musicbrainz data (and what get’s used and what not) and from what I’ve read it’s a work in progress. What seems to be missing is a way to correct or enrich existing metadata the musicbrainz way. For example I’ve a question open regarding release versions (which is important for my use case):
The nice thing about entering data into musicbrainz btw is: one does something for the so called greater good. 