config.yml

OrderActionsForm:
  allow_cancelling: false
  allow_paying: false
  extensions:
    - OrderActionsFormExtension

OrderActionsFormExtension.php

<?php

/**
 *
 */
class OrderActionsFormExtension extends Extension
{
    private static $allowed_actions = array(
        'copyOrderToCart'
    );

    public function updateForm()
    {
        if($actions = $this->owner->Actions()){
            $actions->push(
                FormAction::create('copyOrderToCart',
                    _t('OrderActionsFormExtension.CopyOrderToCart', 'Copy order to cart')
                )
            );
        }
    }

    public function copyOrderToCart($data, $form)
    {
        $form->clearMessage();
        if($order = $this->owner->order){
            $cart = ShoppingCart::singleton();
            foreach($order->Items() as $item){
                // try to add the product if it's still available
                if($product = $item->Buyable()){
                    $cart->add($product, $item->Quantity);
                }
            }

            return $this->owner->controller->redirect(CartPage::find_link());
        }
        $form->sessionMessage(
            _t('OrderActionsFormExtension.CouldNotRestoreCart', 'Unable to add your order to the shopping-cart.'),
            'bad'
        );
        return $this->owner->controller->redirectBack();
    }
}

To make things more robust, you could also store the SKU of a product with each OrderItem. That way you could also restore the Items in cart by SKU (for example if Items were deleted and re-imported and therefore have a different ID, but the same SKU)