Question:
Can I add more cameras to an NVR even though I'm using all of the supported channels?
Answer:
No. You can only add as much devices (cameras) to the NVR as it supports. You can find this information in the data sheet of the recorder.
Ex: You have a DS-7608NI-I2/8P and 8 cameras already added to the NVR (which is the amount it supports). You cannot add any more than 8 cameras/devices since the NVR only supports 8 channels (devices/cameras).
** Screenshot taken from the DS-7608NI-I2/8P's data sheet.