Magento – Configurable products quantity always 0

This is not possible out-of-the box with Magento.

There are a few community extensions on Magento Connect - I have not used them personally. They used to be more prevalent - even Classy Llama had one. There was also one called Package Builder that I cannot find anymore.

So this might be your best bet:

Configurable Bundle by Wizkunde

https://www.wizkunde.nl/magento-2-configurable-bundle.html

Configurable products by Brim

http://www.brimllc.com/2010/12/magento-grouped-products-containing-associated-configurable-products/

If you want to forgo the idea of using bundles, Grouped Products offer the ability to add configurables to the group.

I had the same issue and here is what I've used. It's a 'quick and dirty' script that works with the database directly, so back up you DB before trying it. Also add the table prefix if you have one.

$resource = Mage::getSingleton('core/resource');
$attributes = array('image', 'thumbnail', 'small_image');
$connection = $resource->getConnection('core_write');
$collection = Mage::getModel('catalog/product')->getCollection()
    ->addAttributeToSelect($attributes)
    ->addAttributeToFilter('type_id', 'configurable')
    ->setOrder('sku', 'ASC');

foreach($collection as $product){
    //$product->load($product->getId());//not sure if this is needed
    $simpleProductsIds = $product->getTypeInstance()->getUsedProductIds();
    $q = "SELECT * FROM catalog_product_entity_media_gallery WHERE entity_id = {$product->getId()}";
    $result = $connection->query($q)->fetchAll();
    foreach ($result as $row){
        $q = "SELECT * FROM catalog_product_entity_media_gallery_value where value_id = {$row['value_id']}";
        $valueResult = $connection->query($q)->fetchAll();
        foreach ($simpleProductsIds as $id){
            $q = "INSERT INTO catalog_product_entity_media_gallery SET
                attribute_id= {$row['attribute_id']},
                entity_id = {$id},
                value = '{$row['value']}'
            ";
            $connection->query($q);
            $lastInsert = $connection->lastInsertId();
            foreach ($valueResult as $val){
                $q = "INSERT INTO catalog_product_entity_media_gallery_value SET
                    value_id = {$lastInsert},
                    store_id = {$val['store_id']},
                    label = '{$val['label']}',
                    position = {$val['position']},
                    disabled = {$val['disabled']}
                ";
                $connection->query($q);
            }
        }
    }
    //update media images
    $data = array();
    foreach ($attributes as $attribute){
        $data[$attribute] = $product->getData($attribute);
    }
    Mage::getSingleton('catalog/product_action')
        ->updateAttributes($simpleProductsIds, $data, 0);
    echo "Done with product: ".$product->getId()."<br />";
}

It took about 20 seconds for 80 configurable products and about 400 simple products associated to configurable.
If you have 20k products I don't think it's a good idea to iterate the product collection like I did. You can use Mage::getSingleton('core/resource_iterator'). More about it here.