You can use smart playlists in iTunes and import them to Rekordbox via bridge. The best way is to keep all organizing on one platform otherwise it will get messy very quick. I have one "Rekordbox" folder in iTunes that contains subfolders, playlists and smart playlists. Every time I add a tune I delete the whole playlist library from Rekordbox and re-import the main folder via bridge. Only takes a minute and gives you a lot more sorting options.