At one level Zen cart is just a database with 1,300 php scripts to control input and output of the data. So you could bypass the Zen Cart code and address the database directly from your own script. But I suspect that you would have to write a lot of complex PHP to do so.

I can't help feeling that this is a case of the tail wagging the dog. In essence you're asking how to replicate the order class, and all the stuff that goes around it, when maybe a better approach would be to take advantage of the existing back end processing and instead inject your php page specific information into a clone of the checkout_confirmation page.

But whichever way you choose to do this, it's way bigger than can really be handled in a forum post.