The wItem parameter that you have in the handlers is a reference to the curren Work Item and not to the previous one.
What you have to do is to return a Map<String,Object> in WorkItemA (just like you are doing), then map the results of that Work Item as process variables, and then map those variables (just the one you will need) as input parameters of WorkItemB. In WorkItemB you can get those values using wItem.getParameter(...).