Вы можете "перебить его" с помощью get_posts
и foreach
l oop:
$posts = get_posts([
'post_type' => 'post',
'posts_per_page' => -1, // will grab all
'post_status' => ['publish', 'draft']
]);
foreach ($posts as $post) {
// get post id from $post object
$post_id = $post->ID;
// get the excerpt
$excerpt = get_the_excerpt($post_id);
// there is no "sub title" function in WordPress so we can save it as postmeta
update_post_meta($post_id, 'my_sub_title', $excerpt);
}