plex itunes support is primarily for when you have both itunes and plex server on the same computer.
you can try to copy the itunes.xml file (the one that is your itunes library) to your nas, then put the applicable path into the plex server settings, and see if you can import it then.
unfortunately, there just has not been much work or information provided about itunes support in a long time.
here is an old support article
