The Business Process Model and Notation (BPMN) has emerged as one of the dominant graphical modelling languages for processes in recent years. Its usage is both for conceptual workflow modelling and for specification of executable workflow schemata. Nevertheless BPMN seems to be less suitable for modelling of business processes. Business process models describe production and delivery as well as control of goods and services according to the goals of an enterprise (task level). These characteristics are not covered explicitly by BPMN. In contrast, workflow schemata specify solution procedures for the execution of business tasks (actor level). In this paper a two-stage modelling approach is proposed in order to overcome the semantic gap between business process models and workflow schemata: In a first step a business process model according to the Semantic Object Model (SOM) is created and stepwise refined. In a second step a BPMN workflow schema is derived from an adequately refined business process model according to a metamodel-based schema transformation. The modelling approach is illustrated using the case study of an online auction house.