You can use AudioBus (to play music from another app) or the in-app player as the music source, the output could then be connected to your DDJ-SX, however the SX or Serato itself could not be the source.
Post is closed for comments.
Ok for example I got serato rmt on my iPad I could either plug it via my network or use the usb cable and I could control my music like stopping playing cues fx if am playing a song I could add fx to it using serato rmt instead of using the pad so I can use rmt 1000 like that
It is not for external audio sources.