How can we handle same inventory part for purchasing & manufacturing?

In our setup - we have some parts we both buy (Purchase) and build (Manufacture).  What we figured out is (and this applies to both Apps8 and Apps10)

1. The inventory part type determines your default supply process - but doesn’t lock it in.
2. You can add a Purchased Part to a Manufactured part
3. You can add a Manufactured structure to a Purchased (not purchased raw) part
4. If you normally build something - but need to buy it
   1. Create a purchased part and supplier for purchase part
   2. Create the PO
5. I can’t remember if you can use supply code PO Transit or PO Direct on a manufactured part.  In our scenario, we were always building something we normally buy.  If you need to - you can manually peg the customer order line to the PO
6. If you want to build a purchased part and want to peg the shop order to the customer order line, you have to use supply code Invent Order and manually peg.  You can’t use supply code shop order.

Hope this helps

Yes it is possible. If you create a PO for Manufactured part the system will automatically ask you to create Supplier for Purchase part and by this the Demand code on the PO will be Invent Order.

Following are 2 answers which were posted for a similar question

 I will also go through some of the things discussed here...

1. Yes, An inventory part can be defined with part types manufactured (or manufactured recipe), and purchased 
2. Yes
3. In this case, the system will create 2 product structure headers. One with type “manufacturing”  and the of type “purchasing” part. Manufacturing structure will be used when the part is manufactured, and the purchasing structure is used when the components of the purchased part should be planned, and sent to the supplier with the purchase order.
4. Yes
5. It is possible to have PO transit and PO direct flows
6. Yes

Additionally remember to connect the part to a cost template that has cost buckets for both manufacturing an externally acquired costs. 

For the cost calculation itself you can chose in the setup whether to base the calculation on the manufacturing or externally acquired costs (purchase)