Yes it is, neich78!
Change the ./classes/pdf/HTMLTemplateDeliverySlip.php:
Search for
$carrier = new Carrier($this->order->id_carrier);
and add above
$customer = new Customer((int)$this->order->id_customer);
Then add this line
'customer_email' => $customer->email
to this array
$this->smarty->assign(array(
'order' => $this->order,
…
));
Clear the cache and generate a delivery slip of an existing order, as described above. Worked neat for me.