We have introduced the capability of adding items from the sales order to purchase orders that are still "opened' when proposing a purchase. This reduce the need of creating a new purchase order whenever you propose items from sales order.
Issue a sales order with your regular items
Click purchase (blue button), select purchase item(s) one to one
A propose purchase popup will appear and you can select the supplier for each of the item(s) that you want to purchase
If there are purchase order that are "opened", "add to existing PO" will appear. Click on it and the dropdown list will appear for you to pick the PO that you wish to add the item(s)
Fill up the relevant details and you're good to go
On the purchase order listing page, you'll be able to see the "chain" logo beside the purchase order to indicate there is a SO-PO link to items within the PO
You may use the back to back order purchase report for in-depth information on the order/purchase under reports. Enjoy!