The Business Process Modelling Notation (BPMN) is a graph-oriented language in which control and action nodes can be connected almost arbitrarily. It is supported by various modelling tools but so far no systems can directly execute BPMN models. The Business Process Execution Language for Web Services (BPEL) on the other hand is a mainly block-structured language supported by several execution platforms. In the current setting, mapping BPMN models to BPEL code is a necessary step towards unified and standards-based business process development environments. It turns out that this mapping is challenging from a scientific viewpoint as BPMN and BPEL represent two fundamentally different classes of languages. Existing methods for mapping BPMN to BPEL impose limitations on the structure of the source model. This paper proposes a technique that overcomes these limitations. Beyond its direct relevance in the context of BPMN and BPEL, this technique addresses difficult problems that arise generally when translating between flow-based languages with parallelism.