Hello Karin,
In SRM7 we have a configuration setting for creating the Follow-on document.
SPRO->SRM Server->Cross Application basic settings->Define Object in backend system(PR,Reservation,PO)...
or You can use the Business Add-In BBP_TARGET_OBJTYPE if you wish to implement a different logic for creating documents in the backend system. or use BBP_DOC_CHANGE_BADI ..
Regards,
Sankarbabu