Hi Maesitos!
The NCS software can be controlled by external commands sent by the URL, http//… and the recording started and stopped together with many other commands. However, you need a system that can generate these URLs.
Many cams have their own built in motion detection system as the DCS-942. I think they have a built in software system and also the PIR. To use that signal you need an external output. I am not sure if that is available.
Normally, when connecting cams to NCS or any other surveillance software you use that software’s motion detection system since it is much better.
If you want to use the motion detection system in the DCS you can store the files directly to a NAS or HD, but that is usually not a good solution when you have 5 cams.
I can understand that it can be of interest to use an external motion sensor to start/stop recordings in NCS if the target is out of view of the cameras.
Hm, why complicate life … 
;).
If you have a very special reason for your wish maybe I can help you better.
Regards,
Henrik