It is possible to use Nginx together with the nginx-rtmp module to restream media or a livestream. This can then be used as source for an external live event (a pull event).
When doing this, we recommend using ffmpeg for restreaming.
Keep in mind the recommended encoder settings in this knowledge base article.
If you are restreaming from RTP/RTSP, you may need to force TCP with the ffmpeg option "-rtsp_transport tcp".