Christian,
yes, you can produce any XML (or anything else actually) you need in an ExportMarshaller. The trick would be to override the private void createXML(EObject object) method and implement the hook to produce the additional task at the correct point or to do it from one of the export classes that gets invoked. The task itself will probably not be that hard, it's the rewiring that will require some concentration, since the transitions are marshalled separately, so you you need to jump in at all the correct moments.
