You can drag and drop into mdb all the file formats that mdb uses. These include audio files (ogg and mp3 formats) playlists (m3u and pls formats) and directories. Playlists will be added to the playlist window if they have at least one readable track. Any tracks in a new playlist, that are not in the library, will be added to it.
All audio files in a directory or subdirectory dropped into mdb will be added to the library and the current playlist. Only non duplicate files will be added. The same is true of individual audio file dropped into mdb. In a single operation you can drag and drop any combination of these files into mdb.
If a directory or playlist is involved, the operation may take some time. The section on scanning a directory describes what happens for such operations.