Can tracks managed with iTunes be input to this unit's USB sound card (audio interface)?

Yes.

Windows

Click on the Start menu, select "Run", type "mmsys.cpl" and click "OK". The audio settings window will open.

At the [Playback] tab, select "DJM-900NXS2". Audio signals will be sent to the Channel 1of this DJM. Signals cannot be sent to other channels.

*This cannot be used at the same time with the ASIO driver.

Mac OS

Open [System Preferences] > [Sound] > [Output] and select "DJM-900NXS2" from the list of sound output devices.

Open the Audio MIDI Setup application located in Utilities. Select "DJM-900NXS2". You can change output channel for this DJM by using [Configure Speakers ...] dialog.