This is the solution
function add_tags_products()
{
global $product;
$args = array(
'post_type' => 'product', // your product post type
'posts_per_page' => - 1,
);
$posts = get_posts($args);
foreach ($posts as $post):
setup_postdata($post);
// check to see if the post has any tags
if( ! has_term( '', 'product_tag', $post->ID ) ) :
// create the term
wp_set_object_terms($post->ID, array('nou'), 'product_tag');
endif;
wp_reset_postdata();
endforeach;
}
add_action('init', 'add_tags_products');