Add fields to entityform, when it's created
Yes it's possible  here's the steps on how to achieve that functionality.

1. First you need to alter the entityform and add your custom submit handler

Example:


/**
 * Implement hook_form_alter().
 */
function example_form_alter(&$form, &$form_state, $form_id) {
  if($form_id == 'entityform_type_form') {
    $form['#submit'][] = 'add_new_fields';
  }
}


2. Create your custom submit handler, you need field_create_field(), field_create_instance() in creating your field, define all the details of your field inside the $field and $instance array


/**
 * Submit callback adding custom field when saved entityform type.
 */
function add_new_fields($form, &$form_state) {
  // Create email address field.
  if(!field_info_field('field_email_address')) {
    // Create your new field.
    $field = array(
      'field_name' => 'field_email_address',
      'type' => 'text',
    );

    field_create_field($field);

    // Create field instance.
    $instance = array(
      'field_name' => 'field_email_address',
      'entity_type' => 'entityform',
      'bundle' => $form_state['values']['type'],
      'label' => 'Email address',
      'settings' => array(
        'entityform_type_form' => 1,
      ),
      'widget' => array(
        'type' => 'textfield',
      ),
    );

    field_create_instance($instance);
  }
   // Create voucher code field.
  if(!field_info_field('field_voucher_code')) {
    // Create your new field.
    $field = array(
      'field_name' => 'field_voucher_code',
      'type' => 'text',
    );

    field_create_field($field);

    // Create field instance.
    $instance = array(
      'field_name' => 'field_voucher_code',
      'entity_type' => 'entityform',
      'bundle' => $form_state['values']['type'],
      'label' => 'Voucher code',
      'settings' => array(
        'entityform_type_form' => 1,
      ),
      'widget' => array(
        'type' => 'textfield',
      ),
    );

    field_create_instance($instance);
  }
}


Here's the link of working custom module that i created https://github.com/umandalroald/entityform-custom-callback