Hi MAciej.
Interessting! Maybe for you it is interessting that I "once upon a time" built a working POC of a simulation using jBPM 3, see my Master Thesis: http://www.camunda.com/download/20080201-bernd-ruecker-business-process-simulation-with-jbpm.pdf and two sub sides of http://www.bpm-guide.de/open-source-bpm/ (unfortunately due to some homeopage refactorings a lot of links are broken :-().
Cheers
Bernd