Hi jwalker,
Our stock screens don't contain this functionality, but it can certainly be added via customization. If you're looking for a reference on how to create this functionality, I suggest you have a look the process for generating an invoice from a sales order. I believe that you would need a screen like the one you see after selecting Create Invoice... for selecting which items you want to add to the purchase order.
This does sound like a fairly involved project. If you'd like our help with this, I suggest you reach out to your account manager to discuss having this functionality added by our professional services team. I hope this helps.
--Audisho