I found solution for me. It didn't work with one entity manager factory, but I managed to make it work with two.
Persistence.xml has in both persistence units (nearly) all classes because I use one orm.xml which merges all queries. In code I have following:
EntityManagerFactory emf = Persistence.createEntityManagerFactory( "org.jbpm.persistence.jpa" );
EntityManagerFactory emfTask = Persistence.createEntityManagerFactory( "org.jbpm.myTask" );
Environment env = KnowledgeBaseFactory.newEnvironment();
env.set( EnvironmentName.ENTITY_MANAGER_FACTORY, emf );
env.set( EnvironmentName.GLOBALS, new MapGlobalResolver() );
TaskService taskService = new TaskService(emfTask, SystemEventListenerFactory.getSystemEventListener());
I don't understand exactly why the persistence unit for process and for task are different.
If anyone makes it running with one persistence, let me know.