Right now as a review repo, CB has user-written reviews and reviews imported from the BBC. Is there an option for trusted users (maybe with peer-voting to verify license) to import reviews from other sources licensed under CC licenses? If not, is this something the CB dev team has in mind for the future?
PS: Is CB still importing BBC reviews?
IIRC the BBC wanted to stop doing reviews at all. But maybe they didn’t, in the end, not sure.
I don’t think there’s another way to import reviews at the moment, since CC reviews are quite rare. But if there’s a good source of those, it might make sense to import them.
I have found no music review site that publishes in CC, but I have come across individual blogs of music enthusiasts who have written CC reviews with quality matching that of the reviews imported from BBC Music.
Maybe they have; what a shame. 
I have compiled a list of sites that have CC reviews here on the wiki.
Critiquebrainz does have a write api https://critiquebrainz.readthedocs.io/api/endpoints.html
So if the reviews are under a creative commons licence that allows redistribution anyone could write a bot to copy the reviews.
It might be a good idea to ask first before you start copying the reviews.