Hi Jurgen,
If you dont want to have PR generated, then you can set the config for PR generation to Never.
Plant Maintenance and Customer Service - Maintenance and Service Processing - Maintenance and Service Orders - Functions and Settings for Order Types - Define Change Documents, Collective Purchase Requisition, MRP relevance
So PR will not be generated, then if you need then you can change the PR status from Never to Immi
Else it is only possible via custom code as given by Jogeswara
Regards
Terence