You can display external IP camera stream enabling the camera page, and put ip address in input control. The address depend on your camera, same for credentials

The screenshot is just an example

If your camera cannot be opened in web browser, then it is not possible to capture it in WebUI.

If you use espcam32 and want to use esp3d also on it, you need to use ESP3D V3 with ESP3D-WEBUI V3