Spotify playlists per danceThere is a tool available to synchronize your library with the online library both ways. It is available for Windows and Linux.
The tool allows you to import the dances (not the music itself) from the online library to the genre tags of your own library. It also allows the export your tags to the online library. Finaly you can just add the meta-information (title, band, ...) of the tracks your library to the online library to allow online tagging.
NOTE: Make sure you BACKUP your local library before importing the tags. The tool has been tested, but you never know.
To allow the upload of dances from your library to the online library, the dances should be in the genre tag of the track.
Note: when adding your library to the online library, some random samples will be taken to allow online tagging. No complete song will be uploaded. It takes about 1.5MB per "unknown" track.
You can download the desktop application for library synchronization here for Windows and Linux.