Select or add entity to a form

Question

You can use the entity type to let a user select from among the current set of entities, or an embedded form to let him add a new entity.

My question is, how can you do both (i.e. offer a select field as well as an 'Add New' field)?

My builders are

$builder->add('customer', 'entity', array(
    'class'    => 'TechrescueAdminBundle:Customer',
    'property' => 'label',
));

and

$builder->add('customer', new CustomerType());

but of course, one will overwrite the other if I have both.