Pulling Featured Image from Wordpress Custom Fields into RSS Feed

Hello! I use a Wordpress theme that shows a header image for each article. This is inserted in the post with PHP by a custom field where I enter the URL for the image. I'd like to add a function, so that when a header image exists, it is displayed above the post excerpt in my RSS feed. Similar to this tutorial: [login to view URL] But instead of using the built-in featured image function as show in the tutorial, it would call the custom field header image. When no header image is used on a post, it should also function normally, and not insert any characters or errors. Thanks!