Transcode h264/mjpeg with new resolution as rtsp mjpeg

I have the problem that my ip cam (Dahua) can not stream 640×480 as sub stream to feed my 2N Verso as external camera. It only supports 1280×720 or a 704x?? and 320×288.

I played around with FFmpeg and got it work as http-stream. But the Verso only accepts RTSP streams.

Do I have any chance to get a h264-stream or a 720p mjpeg running as mjpeg (with a resolution of 640×480) like rtsp://ip-cam:554/stream.jpeg?

Thanks and regards Martin