AFAIK, you can't create process variables on-the-fly. If you want Handler1 to create a "new" variable visible from Handler2, then you need to define this variable in the process and to create the output mapping for Task1.
Thanks Esteban. What about in session? Is there a way if handlers know the session?
In 3.x jbpm context I thought we could set whatever new variables and access in other handles..