How to include Related Posts in WP RSS feed

You have to use the_content and the_excerpt_rss hooks. You can do this like this:

function rss_append_footer($content) {
    if(is_feed()) {
        $related_posts_html = ...  // I assume you already can compute it
        $content .= $related_posts_html;
    }
    return $content;
}

add_filter('the_content', 'rss_append_footer');
add_filter('the_excerpt_rss', 'rss_append_footer');