Co-product material with external procurement

Hello,

Our process is in two steps :

1. Mounting step, in which we gather some materials to produce an assembly. Components can be procured externally or in-house.
2. Demounting step, after which we expect to get back each component.

For that, we build two BOM one with positive quantities, and another one with negative quantities (only one "demounted material" is positive). In the BOM with negative quanties, the components are flagged as co-products.

The point is that when we try to check the flag co-product in external procured material masters, we get the following message:

R3 827

Only external procurement is defined for the material.

Is there a way to make this comnination possible by some customizing or by making the message non blocking?

Any help will be appreciated.

Regards,