How to add sound from a microphone to an rtmp broadcast using FFmpeg and output it to YouTube?

Question

YouTube

There is a link to rtmp broadcast - rtmp://domain/live/stream, you need to use ffmpeg to grab the video stream from the broadcast and audio from the microphone and retransmit it to the YouTube server. Preferably without transcoding the video stream (to reduce the load on the CPU).
Why do I need it ?
The broadcast itself will be broadcast in a foreign language, I want to be in the role of an interpreter.