Itunes uses playlists as a filter, but there is still all that music accessible to you - you can do the same with Subsonic, but the browser always lists all the music you have defined in your 'music folder' options. Since subsonic is directory based, meaning it will sort the music in the same way you sorted your music folders, you can now either go ahead and create a directory structure for your individual users (bad idea), or you simply try to either LOAD playlists (under settings --> general there's a folder you can also specify for playlists - not sure how well that import from itunes would go though) or you CREATE playlists for these users and save those.
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