I’ll mark this for the developer’s attention if he has time.
While I can see how this would be useful, this is not a feature that is available in WP without using a plugin and one of the things that I know is that the developer does not generally want to add functionality to the plugin outside of what WP does, with the exception of building interfaces to manage custom fields.
This is possible with the filter hooks that ACF supplies. For example you could create a URL field called images to import where the user could enter the URL(s) of images. Then you can create an acf/update_value filter http://www.advancedcustomfields.com/resources/acfupdate_value/. In your filter you can create PHP and use WP function to get the image from the URL and insert in into the media library. You could then update another ACF field with the ID(s) returned form doing this. You could also delete the original value that was entered into the URL field.
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