If you have advanced understanding of database concepts, you can extract the required records from the appropriate tables, and copy them across. However, how do you reconcile the fact that order numbers may now be duplicated? You'll have to renumber some things accordingly. Your customers' email receipts will point to incorrect places, but at least their My Account area will have the new details.

Database schema reference: http://www.zen-cart.com/wiki/index.p...#Order_History