I think the USB port is meant for a drive / stick with files on it. You could possibly connect the iPad to the HDMI input with the Apple Multiport A/V adapter, but you lose the ability to charge the device while it's in use.
Another option is to connect the UDP-203 to the LAN and use it as a DLNA renderer. You'll have to download an app like mConnect app to stream music. I've tried it with files stored on my Mac-based server (running Kodi) and Qobuz. Works and sounds great.