Hi Felix,
OK, I know you want to use jBPM as a FSM. But the StateActivity is not a place to change the state of business information. You should understand the usage of StateActivity, it is used to wait for a external message from outside the process engine.
If you want to maintain the state of business information, you could use the JavaActivity or ScriptActivity, both of them can do what you want, and they can go on execute.