Not sure why you would want to copy the content to the post body field when you can add the field's content directly to the post body without the user's populating the custom field.
Can you provide a little more information on this one ?
No worries, I would highly recommend doing it the correct way by placing the content in the post content area directly. This will help to declutter the database and not have duplicate content on your website in the event that you decide to display this content and the post content area.