Beets update empty fields

My files are tagged manually very good before i’ve used beets.
How can i fill the empty tags (eg musicbrainz id) without modifiying the current existing tags?

Can I ask why you want the musicbrainz ID if you don’t want musicbrainz to influence the tags?

because tags like genre are very personal taste. I don’t want this to be overruled

Hmm, I think import -W (don’t write tags on import) and then beet update should do what you’re asking. Then once the database is filled with the correct info, finish it off with beet write to write the database tags to your actual music files.

See the update command and the write command