Hi Nitisha,
One of the solutions to your requirement could be like that:
1. You already have characteristic ARTNR created in your operating concern (it's a system created characteristic) and it caries material code for material created in SAP.
2. Create a user characterictic e.g. WWMAT. Make sure that in validation you specify "No check".
This characteristic should be CHAR and of length that can accomodate both SAP created material codes and external material codes.
3. In config transaction KEDR you define derivation rule for WWMAT. The rule should be MOVE and source field ARTNR.
4. This way for every SAP created material WWMAT will be populated (in addition to ARTNR).
5. In your FI journals (posting to profitability segment) bringing revenue and COGS from external system you populate external material code in WWMAT.
6. You'll need to ensure consistency of external material code in FI journals as this field in CO-PA is not validated.
7. You base you reporting in CO-PA on field WWMAT (not ARTNR) as this field will contain both internal and external material codes.
This way you can bring external materials to CO-PA without creating material numbers in SAP.
Regards,
Szymon