So you want an exit which enabled you to post credit memo based on the next run selected (not paid, just selected documents ?) and that document should also been added to the run.
So the exit is executed after standard extraction execution and before next step starts allowing to add records after standard extraction (I have some doubts...)
Nevertheless, you could start looking at BTE PROCESS 00001820 (Display BERP Activation FIBF) or other BTE available (1810-1860).
Regards,
Raymond