I have a DS-7608ni-i2/8p which have 8 Hikvision poe cameras onboard.
If you already have 8 cameras connected to your 8-channel NVR it's full - you can't add another.
If however you mean that you have 8 PoE ports, not all used, and you want to connect a LAN-connected (WiFi) camera, then this is what to do:
Use SADP or the camera web GUI or DHCP reservations to set a fixed IP address on the WiFi camera.
On the NVR web GUI, under System | Camera Management, choose the unused PoE channel that you want the WiFi camera to appear on.
Select the channel and use Modify to change the mode to Manual instead of Plug&Play
Change the IP address of the channel to match that you have set on the camera.
Set the camera password and click OK.
At that point, the camera should appear in Live View.
Be aware that WiFi-connected cameras can be quite troublesome, with connection drops and jerky Live View and Playback.