Magento: add to email template custom block

It's can by:

{{block type='core/template' area='frontend' template='email/my_email_block.phtml' order=$order shipment=$shipment }}

Then in the template file email/my_email_block.phtml
-> You can access the Shipment object by using $this->getShipment().
-> You can access the Order object by using $this->getOrder().