Hi,
You should make the process call a web service of your system, or some other means of communication i.e. ejb, jms etc. This can be easily implemented within a work item.
Another approach could be to pass an object to the process which allows to add a listener to it and then make the process to trigger the event, when it reaches the desired state. In more detail, you add a listener from your system to that object, pass the object as input parameter to the process and the process triggers the event that in turn will call all the listeners registered to the object i.e. one of them should be a listener of your system.