Additional properties to entity Field Type in a form in Symfony2

In Symfony2, is there a way to map more fields from an entity to the option tag of a select dropdown generated from a form which is based on an entity?

I currently have something like:

    $builder->add('creditcard', 'entity',
        array( 'label' => 'Credit Card',
            'required' => true,
            'expanded' => false,
            'class' => 'Acme\Bundle\Entity\CreditCard',
            'property' => 'display_text',
            'multiple' => false,
            'query_builder' => function(\Acme\Bundle\Repository\CreditCardRepository $er)  {
                return $er->createQueryBuilder('b');
            },
            'mapped' => false,
        ));

This works just fine, but I would like to generate something like:

<option value="id" string_mapped_from_field1="value_of_field1">display_text</option>

Thanks!