If you right click (control+click) on a playlist in rekordbox, select "save playlist info" and change the format in the dropdown from "txt" to "m3u8" you can then open that playlist file in iTunes and it'll appear in the playlists bank in iTunes. Obviously this doesn't export the tracks as well... the media where the tracks are located needs to be connected to play in iTunes.
If you want to export the actual tracks as well, the only way I know to do this 'easily' is to format a flash drive, export the playlist from rekordbox to the flash drive, then search the flash drive for all files (*) and copy all of the music files across to a new folder so they're all in the one place (as opposed to scattered throughout the 'contents' folder that rekordbox exported to). Then you can add those tracks to iTunes to create the playlist - I would only bother with this if you store you music on an external HDD and want to access the music when your HDD is disconnected.
Hope all that makes sense...
