Can I input the music tracks managed in iTunes into the USB sound card (Audio interface) of the unit?

Yes, you will be able to do so if you install the dedicated driver software of this unit on your PC.
-Windows
Display the [Run] dialog box from the [Start Menu], enter [mmsys.cpl], and then click [OK]. The audio settings window opens. Select [PIONEER XDJ-RX2] in [Sound playback] in the [Audio] tab.
-Mac
In [Select a device for sound output] on the [Output] tab in [Sound] of [System Preferences], select [PIONEER XDJ-RX2] from the sound devices.

If MIXER MODE is set to XDJ-RX2, the sound will be input to channel 1 of this unit. It cannot be input to other channels.