Thanks Maceij,
I went with a work-around:
I made transitions to the tasks match tasks names: if a task is called 'task-1', then transition is 'to-task-1'. Holding the pattern, i can do:
processEngine.getHistoryService().createHistoryTaskQuery().executionId(executionId+".to-"+activityName).state(Task.STATE_COMPLETED).uniqueResult();
where activityName is 'task-1'
Thanks again for help!