Would it be possible to use the same code with some modifications to get the value of a field and use it as the meta-key _yoast_wpseo_focuskw? Something like this:
add_action('acf/save_post', 'automatic_keywords');
function automatic_yoast_keywords($post_id) {
if (get_field('_yoast_wpseo_focuskw', $post_id, false)) {
// get unformatted value
// we did it above too, no point formatting the value for this
$image_id = get_field('_yoast_wpseo_focuskw', $post_id, false);
update_post_meta($post_id, 'name', $meta);
}
}