Thanks Prashanth
I appended this field PRGRP to the extract structure.
At the end, after refreshing RSO2, I see the new field in the datasource.
Additionally I built a workaround:
I built a new datasource based on a DB-view including the key fields, BUKRS and PRGRP as data fields.
This would have been the workaround if the initial efforts would have failed.
thanks