In Apps 10, we have done a configuration - Event Action to fetch the correct routing from the Purchase Authorization Rules and apply it to Sub Contracts / Service Contracts as Approval Routings (At the bottom task pane - paper clip symbol where you can see attachments etc_). We had doc approval templates with the same ID as the PO routing templates. I am sure with Business Process Automation (BPA) module it will be easier than in Apps 10. It took us several days to get it correct.