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-450". Audio signals will be sent to the Channel 1of this DJM. Signals cannot be sent to other channels.
You will be able to select the input channel from CH1, CH2 or AUX if you update the following software:
・Firmware: Ver.1.02 or higher
・Driver software (DRIVER FOR WINDOWS): Ver1.101 or higher
*This cannot be used at the same time with the ASIO driver.

Mac OS
Open [System Preferences] > [Sound] > [Output] and select "DJM-450" from the list of sound output devices.
Open the Audio MIDI Setup application located in Utilities. Select "DJM-450". You can change output channel for this DJM by using [Configure Speakers ...] dialog.
You will be able to select the input channel from CH1, CH2 or AUX if you update the following software:
・Firmware: Ver.1.02 or higher
・Driver software (DRIVER FOR MAC YOSEMITE): Ver. 1.1.2 or higher
　　　　　　　(DRIVER FOR MAC SIERRA AND EL CAPITAN): Ver. 2.1.2 or higher
In this case, please set Mac OS [System Preferences] to default.

Pioneer DJ provides the latest firmware and driver software on its web support page. Get them this page.