Purchase orders and Item Planning

Okay maybe I'm just being stupid, but when we are doing our planning we often start with an excel spreadsheet which we use as our purchase order and then once the merchandise arrives at the store we create the item using the vendor UPC and then do a purchase order and receive it. I really want to be able to create a purchase order before the merchandise arrives, but the only way I see to do this is use a dummy item lookupcode until the merchandise arrives and then switch to the vendor UPC once it arrives. What a pain in the a.. though. Does anyone know of a way to enter the items in a planning mode then create purchase orders once I am completed with the planning and the items have arrived and I know the UPC? I realize I am talking about a planning system but don't want to make a huge purchase and have to integrate yet another system, but I really am spending to much time keying data in to many places. I don't really like doing it in excel and would prefer to have it all one system. Any help here would be appreciated. I'm kind of surprised Microsoft doesn't have a top-down, bottom-up item planning system/OTB system.