Nothing special. Just add the same way as you add IP camera. Then you will be prompted to add all the channels or select some channels from the DVR. Make sure also ports 554 are accessible from the NVR side (in case the DVRs are behind firewall/router).
P.S. Sorry, I didn't see you're talking about Netvision DVR. I'm not sure what protocol uses Netvision DVR. In general you can add channels to a Hikvision NVR using RTSP streams (if Netvision supports RTSP).
