Hi Klaus,
we had the same requirement. We use jbpm 4.4 and create a histroy variable on process start:
String userId = ...
ProcessInstance processInstance = ....
executionService.createVariable(processInstance.getId(), "initiatior", userId, true);
The "true" makes this variable a history variable. Especially useful as you could query the database for it (we do it outside of jbpm), even for ended processes. As we also need the initiator in a swimlane during process intatiation, so we additionally pass this value as a regular process parameter in the startProcessInstance method.
Cheers,
Charles