Today, a company's service processes are interwoven with external services and customers. In these cross-company service networks, partners are interchangeable and market conditions may shift rapidly. Thus, a need for business process optimization arises, though conventional approaches often cannot be used as company's IT is not process centered. To achieve business process optimization in existing distributed infrastructures, we propose the modular aPro reference architecture, which builds upon existing components. Based on the architecture, we develop a maturity model for process optimization, which may be used to assess the need for change and select necessary measures.