Thnx again Esteban!
It seems that the class WorkflowProcessInstance cannot use the methods get/setVariable. Googling on it brings me e.g. to the v3 of jbpm showing that only ContextInstance can use get and set.
Which brings me to the question if "kcontext" can be used in a DRL file without imports. If not, what should be imported and must kcontext be declared in the drl file?
I still have not found a way to access, eval and change process variables in a drl file...