On the product Inventory tab, just check both routes Manufacturing and Buy.
If you use the reordering rules, you can set the preffered route. This will be used to create order if you run the scheduled action. Be aware that you need to assigne a vendor on the product (tab Purchase) in order to automatic get Purchase requisition or orders. Thhis will also be filled with your first manual purchase order for the product. And you need a BOM for the manufacturing route.
In this scenario I would recommend to stop the scheduled job to create orders, and instead use the transaction from Inventory-> Operation ->Replenishment, where you an choose manually to manufacture or buy.