This is possible, but not necessarily easy. You need to find a utility that will receive the RTSP stream from the camera, and offer if as a virtual webcam to the WebRTC application. I’ve done this using vMix, which is a heavyweight commercial video production application. OBS Studio is similar, and open source, but lacks the virtual webcam capability. Most virtual webcam tools cannot use an RTSP stream as a source.
The more difficult approach is to have one application display the RTSP stream, and use another application to screen capture that like a rtmp server which at rtmp can use IP camera, offering it as a virtual webcam. VLC couple play the stream easily. SparkoCam could provide the virtual webcam. The video quality usually suffers using this approach.
I can think of a third approach that could be promising. It involves using the production quality NDI streaming format for passing video over IP. Use VLC to play the RTSP. Install Newtek’s free NDI plugin for VLC. This allows VLC to stream the playback as an NDI stream. Then use one of their other NDI utilities to convert that NDI stream to a virtual webcam. I suspect this could be done on the same host.