Плагин Wordpress для регистрации на курс

Как упоминал @Hughes, вы не можете использовать wpcf7 и просто подключаетесь к нему, чтобы вставлять пользовательский пост в каждый запрос.

// Hook on wpcf7
add_filter( 'wpcf7_mail_components', 'do_on_cf7_submit', 50, 2 );
function do_on_cf7_submit($mail_params, $form = null) {
    // Empty post content
    $content = '';

    // set post content if field not empty
    if ($_POST['field-name'] != '') {
        $content .= 'Field Name Label: '.$_POST['field-name'] ;
    }


    // insert post if content not epmty
    if ($content != '')  {
        insertQueryPost($_POST['email'], $content);
    }  

    // allow cf7 to do his stuff
    return $mail_params;
}

// insert custom post type "query", don't forget to setup your custom post type first
function insertQueryPost($title, $content) {
    // insted of proper post slug, just make a hashed slug, when setting custom post type, set it to not public and not search-able
    $t = time();
    $thash = md5($t);

    $my_query = array(
        'post_title'    => wp_strip_all_tags( $title ),
        'post_content'  => $content,
        'post_type' => 'query',
        'post_name' => $thash,
        'post_status'   => 'publish',
        'post_author'   => 1
    );
    $data = wp_insert_post( $my_query );
}