add_filter('wpcf7_special_mail_tags','wpcf7_currentuser_shortcode_handler',10,2);

function wpcf7_currentuser_shortcode_handler($output,$name) {

    global $current_user;

    if($name=='currentuser') {
        wp_get_current_user();
        $output .= $current_user->name . "\n";
    }

    return $output;
}