Add Attribute to Custom Collection in Admin Grid

I have a 3rd party extension which provides a grid. Now I want to extend the grid with a simple Magento attribute. It looks so easy but I can't get it to work!
I modified the function _prepareCollection() and wanted to add an attribute to the selection. Unfortunately all attempts failed.

That is my most promising solution (added before "setCollection") but it doesn't work either:

    $collection->getSelect()->join( array( '_atr'=>'eav_attribute'), "_atr.attribute_code = 'second_sku'", array());
    // $collection->addAttributeToSelect('second_sku');

Thats the (original) full function:

    protected function _prepareCollection() {
    $orderId = $this->getOrder()->getid();

    $collection = mage::getResourceModel('Purchase/OrderProduct_collection')
                    ->addFieldToFilter('pop_order_num', $orderId);

    //only products not supplied
    $collection->getSelect()->where('(pop_qty - pop_supplied_qty)  >0');

    //add image picture
    $smallImageTableName = mage::getModel('Purchase/Constant')->getTablePrefix() . 'catalog_product_entity_varchar';
    $collection->getSelect()->joinLeft($smallImageTableName,
            'pop_product_id=`' . $smallImageTableName . '`.entity_id and `' . $smallImageTableName . '`.store_id = 0 and `' . $smallImageTableName . '`.attribute_id = ' . mage::getModel('Purchase/Constant')->GetProductSmallImageAttributeId(),
            array('small_image' => 'value'));

    //join with product
    $productTableName = mage::getModel('Purchase/Constant')->getTablePrefix() . 'catalog_product_entity';
    $collection->getSelect()->joinLeft($productTableName,
            'pop_product_id=`' . $productTableName . '`.entity_id',
            array('sku' => 'sku'));

    $this->setCollection($collection);
    return parent::_prepareCollection();
}

Any help will be much appreciated! Thank you!

EDIT:
If I add my code to the original function I'll get following select:

SELECT `main_table`.*, `catalog_product_entity_varchar`.`value` AS `small_image`, `catalog_product_entity`.`sku` FROM `purchase_order_product` AS `main_table` LEFT JOIN `catalog_product_entity_varchar` ON pop_product_id=`catalog_product_entity_varchar`.entity_id and `catalog_product_entity_varchar`.store_id = 0 and `catalog_product_entity_varchar`.attribute_id = 109 LEFT JOIN `catalog_product_entity` ON pop_product_id=`catalog_product_entity`.entity_id INNER JOIN `eav_attribute` AS `_atr` ON _atr.attribute_code = 'second_sku' WHERE (pop_order_num = '1') AND ((pop_qty - pop_supplied_qty) >0);

As you see the select "works" but the attribute is not part of the selection. I already tried addAttributeToSelect and addFieldToSelect but both throw an error.