Combine Orders is a pretty smart extension, which utilizes OpenCart API to combine multiple orders at once. It takes order with the highest ID (base order) from the list of selected orders and uses it as a source of basic information: customer info, shipping and payment methods etc. 

All products and their options from selected orders are re-added to shopping cart and then all shipping, payment and total data recalculated accordingly. 

It works similar to standard order editing, but in silent mode. You just need to select orders and click Combine button!