You could try using a drools.session.conf file, in which you put something like this:
drools.workItemHandlers = CustomWorkItemHandlers.conf
Then in CustomWorkItemHandlers.conf you put in entries:
"Task1": new com.xyz.TaskHandler1(),
"Task2": new com.xyz.TaskHandler2()
jBPM picks up these definitions and adds them itself without having to do so programatically.