I expected that the transformation of a BPMN diagram to a WF workflow would be accomplished by mapping each BPMN Process to a Sequential Workflow. But, there is a structural difference between the BPMN diagram and the Sequential Workflow. The difference is that the Sequential Workflow is block-structured (i.e. all paths from a split or branch reconverge at a single node), while the BPMN diagram is a free form graph. The arbitrary sequence flows allowed in BPMN are similar to GOTO statements. Given that Sequential Workflows does not have a GOTO activity (see a related thread from MSDN forums), the transformation is not possible without redrawing (re-engineering) a semantically equivalent block diagram.
The problem is equivalent with the problem of transforming BPMN to BPEL. Bruce Silver wrote a useful post on the topic and Yi Gao (Chief Architect of eClarus Software) wrote an interesting white paper. From these sources I copied an example BPMN diagram that presents the problem of block-oriented representations (like Sequential WF and BPEL).
But, what if we map the BPMN diagram to a State Machine Workflow? Then we can solve at least the GOTO problem. We can map each task and each gateway of the BPMN to a State activity, which contains a single StateInitializationActivity, which executes its contained activities when entering the state. The State Machine Workflow diagram is the following:
Each StateInitializationActivity which corresponds to a BPMN task contains two childs activities: one for the implementation of the task and a SetStateActivity which causes the transition to the next state. Each StateInitializationActivity which corresponds to a BPMN gateway also contains two child activities: an IfElseActivity to evaluate conditions and again a SetStateActivity which causes the transition.