To add items to a Purchase Order directly from a project, you will want to do so from the project. 

You can either do so from the item status view, or by Viewing the Bill of Materials. You can click on the items in the Item Status view, Add to Order, then select the PO you want to add to. For more information check out updating item status.

Also check out setting items as Ready to Order to take advantage of ordering across multiple projects and adding more process within your ordering operations.