You will find what you need in here http://docs.oracle.com/cd/E24152_01/Platform.10-1/ATGPersProgGuide/html/s1401partiiscenariosmoduleprogramming01.html, since scenarios uses JMS you will be able to call NON ATG applications inside an scenario without any problem.
My understanding is that the events are fired from non-atg system.
If that is the case then have a listener to listener to those events from non-atg system and fire a DMS custom event from the listener.
Now the custom event appears in the ACC to create the scenario which is fired based on the event from non-atg system.
Refer ATG guide to create custom events:http://docs.oracle.com/cd/E24152_01/Platform.10-1/ATGPersProgGuide/html/s2302addingcustomevents01.html