RSS Feed Sticky Post?

You can try the following:

/**
 * A sticky feed post from a given category
 *
 * @see http://wordpress.stackexchange.com/a/176706/26350
 */

add_filter( 'the_posts', function( $posts )
{
    if( is_feed() 
        && $tmp = get_posts( array( 'category_name' => 'feed', 'posts_per_page' => 1 ) ) 
    )
        $posts = array_merge( 
            $tmp, 
            wp_list_filter( $posts, array( 'ID' => $tmp[0]->ID ), 'NOT' ) 
        );

    return $posts;
} );

where we prepend a given category post to the feed. Here you have to adjust the feed category slug to your needs. We also make sure that the sticky post isn’t duplicated.