I wanted to import all my existing M3u playlists into MB in one go. Searching the forums suggested drag and drop but this only worked for one file at at a time.
Because MB supports M3u as well as mbp format I came across a quick fix.
1. In MB options set the Playlist type to M3u. Copy all your M3u files to Music/MusicBee/Playlists. You might have to edit the M3u files if the path is incorrect.
2. Run MB and all your playlist will be in the library.
3. Go to MB options and set the Playlist type back to Mbp. MB will convert all your playlist files to Mbp format.
4. Exit MB.
5. Run MB again and all your playlists are in the library again with the source files in Mbp format.
This saved me a ton of time