'uc_order_product' => array(
  'label' => t('Order product'),
  'base table' => 'uc_order_products',
  'controller class' => 'UcOrderProductController',
  'metadata controller class' => 'UcOrderProductMetadataController',
  'entity keys' => array(
    'id' => 'order_product_id',
    'label' => t('Order product'),
  ),
  'bundles' => array(
    'uc_order_product' => array(
      'label' => t('Order product'),
    ),
  ),
  'view modes' => array(
    'full' => array(
      'label' => t('Normal view'),
    ),
    'cart' => array(
      'label' => t('Cart view'),
    ),
  ),
  // Entity API callbacks.
  'access callback'   => 'uc_order_order_product_access',
  'save callback'     => 'uc_order_product_entity_save',
  'deletion callback' => 'uc_order_product_delete',
)

function YOUR_MODULE_entity_info_alter(&$entity_info){
   if(isset($entity_info['uc_order_product'])){
     $entity_info['uc_order_product']['fieldable'] = TRUE;
   }
 }