Thanks for your help.
I did that before, like this:
ksession.getWorkItemManager().registerWorkItemHandler("MyServiceTask", new MyServiceTaskHandler());
I also wrote the handler class. However, it is still looking for the wrong handler.
When I open the bpmn file in eclipse and check for the connected action entry for this particular task, it shows
org.drools.process.instance.impl.WorkItemImpl workItem = new org.drools.process.instance.impl.WorkItemImpl();
workItem.setName("Send Task");
When I change the setName function to MyServiceTask and reopen the bpmn file, it changes it back. So it must fetch this from somewhere ...