Dimox

function add_to_rss($query) {
	if (is_admin()) {
		return $query;
	}
	$include = array();
	if ( $query->is_main_query() && is_feed() ) {
			$posts = get_posts(array(
				'post_type' => 'post',
				'meta_query' => array(
					array(
						'key' => 'добавить_в_rss',
						'value' => '1',
						'compare' => '=',
					),
				),
			));
			foreach($posts as $post) {
				$include[] = $post->ID;
			}
			$query->set('post__in', $include);
		}
	return $query;
}
add_filter( 'pre_get_posts', 'add_to_rss' );