Hi Amin,
Why are you doing that? the main idea of using work items is that you can copy information from the process scope to the work item scope using parameter mappings. If you define what pieces of information you will need inside the work item handler you can do the mappings and the engine will copy those values for you. You don't need to have the session to retrieve the process variables.
Take a look at the examples here: https://github.com/Salaboy/Drools_jBPM5-Training-Examples
inside the jBPM5 directory for more information.
Look at the processes definition to see how to move information from the process scope to the workitem scope.
Greetings.