if (is_feed()) {
	function feedAppender() {
		global $post;

		$content = get_the_content(); // the actual content of the post
		$appendedContent = "test"; // whatever you want to append to the post

		$content = $content . $appendedContent; // Da appendizer !
		return $content; // Ofcourse return it, else you'll get a blank !
	}

	add_filter('the_content','feedAppender'); // Make this happen when the_content() is being called
}