Can iTunes tracks be input to USB sound card (audio interface) of this unit?

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 "INTERFACE2".
*This cannot be used at the same time with the ASIO driver.

Mac OS
Open [System Preferences] > [Sound] > [Output] and select "INTERFACE2" from the list of sound output devices.
Open the Audio MIDI Setup application located in Utilities. Select "INTERFACE2". You can change output channel for this unit by using [Configure Speakers ...] dialog.