OK after a bit exploring i found a kind of solution by using BpmnParse-Class.
For whom it may interest: I just had to "fake" a Deployment to make it work. Just create a new BPMN-Parse like this:
new BpmnParse = new BpmnParse(new BpmnParser(new ExpressionManager(), new DefaultBpmnParseFactory()))
When using in Tests or elsewhere also make sure that Context.getProcessEngineConfiguration is not null. I first used the Parse in a JUnit Test with an Activity Rule and got a NullPointer. So i just set the Configuration with the static setter.
Now I can at least identify possible predecessors and successors, but by checking active tasks before and after completion the new created follow-ups can be identified (ok, not if you have parallel multi-instance sub processes but I dont have them) =D
