$email=...;
$customer = new Customer();
$authentication = $customer->getByEmail(trim($email));
if (!$authentication OR !$customer->id)
    //Старый код (создаем акк)
else
{
    $cookie->id_customer = intval($customer->id);
    $cookie->customer_lastname = $customer->lastname;
    $cookie->customer_firstname = $customer->firstname;
    $cookie->logged = 1;
    $cookie->passwd = $customer->passwd;
    $cookie->email = $customer->email;
    if (Configuration::get('PS_CART_FOLLOWING') AND (empty($cookie->id_cart) OR Cart::getNbProducts($cookie->id_cart) == 0))
        $cookie->id_cart = intval(Cart::lastNoneOrderedCart(intval($customer->id)));
}