コード
themes/functions.php
add_action('init', 'nobita_add_tag_to_page');
add_action( 'pre_get_posts', 'nobita_merge_page_tag_at_tag_archive' );
function nobita_add_tag_to_page() {
register_taxonomy_for_object_type('post_tag', 'page');
}
function nobita_merge_page_tag_at_tag_archive( $query ) {
if ( $query->is_tag== true && $query->is_main_query() ) {
$query->set('post_type', array( 'post', 'page', 'nav_menu_item'));
}
}
http://ja.forums.wordpress.org/topic/12991?replies=6(@source)