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-250MK2".
*This cannot be used at the same time with the ASIO driver.

Mac OS
Open [System Preferences] > [Sound] > [Output] and select "DJM-250MK2" from the list of sound output devices.
Open the Audio MIDI Setup application located in Utilities. Select "DJM-250MK2". You can change output channel for this DJM by using [Configure Speakers ...] dialog.

You can select channels (CH1, CH2, AUX) to input audio data from your computer. See the Operating Instructions (Changing the Settings－About the setting utility software) for details.