Convert Order to Quote and Load to Current Cart

@user1240207

Please use the below code. It will may help you.

Mage::getSingleton('checkout/session')->setQuoteId($_quote->getId());

Code convert an order to quote please used below:

   $quote = Mage::getModel('sales/quote')->load($order->getQuoteId());
    if ($quote->getId()) {
        $quote->setIsActive(1)
            //->setReservedOrderId(null)
            ->save();
        Mage::getSingleton('checkout/session')
            ->replaceQuote($quote);
           // ->unsLastRealOrderId();
    }

Here the process:

Get Current quote

$currenQuoteId=Mage::getSingleton('checkout/session')->getQuoteId();

Convert Order to quote

$OrderQuote = Mage::getModel('sales/quote')->load($order->getQuoteId());

Active Quote

if ($OrderQuote>getId()) {
    $OrderQuote>setIsActive(1)
        //->setReservedOrderId(null)
        ->save();
}

Merge your current quote and order reverse Quote

if ($OrderQuote->getId() && $currenQuoteId != $OrderQuote->getId()) {
    if ($currenQuoteId) {
    $OrderQuote->merge(Mage::getSingleton('checkout/session')->getQuote())
        ->collectTotals()
        ->save();
    }

set quote to checkout session

Mage::getSingleton('checkout/session')->setQuoteId($OrderQuote->getId());

 if (Mage::getSingleton('checkout/session')->_quote) {
            Mage::getSingleton('checkout/session')->delete();
        }
       Mage::getSingleton('checkout/session') = $OrderQuote;
    } else {
        Mage::getSingleton('checkout/session')->getQuote()->getBillingAddress();
        Mage::getSingleton('checkout/session')->getQuote()->getShippingAddress();
        Mage::getSingleton('checkout/session')->getQuote()->setCustomer(Mage::getSingleton('customer/session')->getCustomer())
            ->setTotalsCollectedFlag(false)
            ->collectTotals()
            ->save();
    }

The previous answer only works if the quote is still available in the DB. I ran into a situation where this was no longer the case, so this code doesn't work:

$quote = Mage::getModel('sales/quote')->load($order->getQuoteId());
$quote->getId() => This is empty, in fact the whole model is empty!

Instead, we can use the Mage_Sales_Model_Convert_Order to convert an existing order back into a new quote using the toQuote(Mage_Sales_Model_Order $order) method. It takes an order as input and outputs a new quote object.

You do need to call extra functions for converting the billing/shipping address, items, etc. For each object you will find a function in this class.