This is not recommended or the manual would have mentioned it. http://www.ppsmanager.com/PPSM_2012/...oughAssist.pdf
You can call or email their website for more information. There are alot of devices out there and whoever told you this might have been confused. The device can be connected using some existing pieces of the vent circuit but is not designed to be used with the vent inline. Some reps are just sales people and don't ever realize airway tubes such as trachs are 15 mm for universal fitting of most devices.
Other devices such as the MetaNeb and IPV can be used with a ventilator but have valves or a pressure relief manifold and specific instructions to prevent over pressuring the lungs. http://tlforrette.com/IPV%20Clinical...on%20Guide.pdf http://www.metaneb.com/index.html
But both of these devices are for secretions although they do not have the same mechanism to assist a cough for clearance.