This topic has been thoroughly covered in previous threads. For example, here:
Your CCTV cameras are powered by 12VDC power supply. You can continue to use it to power the cameras by adding a 12V-to-5V converter (such as a dashcam power kit) at each camera. Your idea of feeding 5VDC over the existing CCTV wiring may or may not be successful, depending on the length of the wiring runs, the gauge of the wire, the phase of the moon, etc. That approach has worked for some, but not for others. No harm in trying.