Functional Area (FKBER) as a COPA characteristic

We have implemented Account Based COPA. We have also written Functional Area substitution rules to derive functional area in accounting. Functional area is correct in Accounting (BSEG).

However, when I want to define the field FKBER as a characteristic for COPA, this is not available. I have looked at notes 410258, 422397.

Any idea how to make the field FKBER available as COPA characteristic?

Regards,