Is it possible to add new column IsPreOrder to OrderItem, so we can trace the product was pre-ordered at the time of invoice? This will help in further development of pre-order functionality.
You can add a new column in the order item table. But for this, you would need to do some customization.
Yes, this can be added with code customisation. I feel it is worth to add this field on OrderItems in New version of Nopcommerce to keep track of it as Pre-order product. At present, it is only on the Product and appears at the time when we add to cart.