Usherr finds your existing collection where it is: nothing is moved or changed.
In Broadcast: Server, Kinds of media, Music, Import music.
Usherr reads your music folder (Artist/Album/tracks works best), looks each album up, and adds what it recognises, where it is. No file is moved, renamed or changed, and only the albums it found are wanted, so nothing starts downloading.
Press Start scanning. It looks albums up one a second, so a big collection takes a while; you can leave the page and come back.
Added, already in your library, and not matched, with the reason and the folder. Rename a folder and scan again to fix one: scanning again is safe.
Every artist found is in Library, Music.