How can I add ítems to cart in Magento 1.9 but in specific store?

Question

hello I have 4 stores configured in magento 1.9, but I need to add items to the cart in a specific store with ID 8, I have tried the following code but it does not work.

$cart = Mage::getSingleton('checkout/cart')->setStoreId(7);
$cart->init();
$cart->addProduct($productId, array('qty' => $qty));
$cart->save();
Mage::getSingleton('checkout/session')->setCartWasUpdated(true);

I hope you can help me, thanks