I think it should be doable via a custom code. That URL field looks like it is a custom field of some sort. All I need to know for this is what is the field name to get the URL, or if it is not a custom field, then how to get that URL programmatically.
Because this is not a built-in feature of WordPress there is probably an API for the plugin adding this feature. There is probably a function of some sort which can return the external URL based on the media attachment ID. If you can find that function (maybe by asking the plugin developer?), then I can add it to a custom code, which should take care of it.