Yes it is possible to port forward to a camera connected directly to the NVR. The NVR will pass the data directly to the camera, however the IP camera will need to have an IP address in the same range as the network router.
If your router has the IP address of 192.168.0.1, ensure your cameras are also in the 192.168.0.XXX range.
Confirm you can connect to those cameras from another device on the network. If this is successful, simply configure your port forwarding as normal.