Hi Jacques,
You can certainly do this by placing a button on the purchase order screen that will display the home -> items screen in a pop-up using the "display pop-up" action; this would enable you to modify your items list with out switching screens per-se, but if you wish to go further and open the item screen on the condition that a typed item isn't found in the item list, I do believe it is possible to do (and our customization guru Michael Melo may correct me on this) but may be quite complex from a customization perspective. Essentially you would have to add a new option in the drop down which would show the desired "new item" screen in a pop up. I can look into this if you would like, but if you want detailed premium support paid customization assistance on how to get this functionality, please contact Michael, and he will assist you.
Regards,
Denis