By default they are not read only 
Sorry there’s no plans for that too many side effects and settings to handle all cases, what about deletion on the server? Import as read only, then import all but X or Y.
Next version have a skip duplicate option in the import dialog, so it’s just a matter of import all with that option to import the missing ones.
Furthermore if your server returns real filenames you can use smart playlists on Symfonium to already built playlists based on the folders. [Wiki] Smart filters