I have this question, but:
- Operating system doesn't matter
- Being opensource doesn't matter
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it is very easy to use and the default settings should be fine in most cases. just add the desired inputs and you are good to go. Take a look at their website.
you would have to record your voice seperately and then combine with one of the commands described in this superuser post. Simplest would be (on linux)
ffmpeg -i video.mp4 -i audio.wav
But that might not work best or be good in any way so evaluate the descibed options yourself.
I would personally use OBS as it is easy to get working and offers many more features like overlays and filters etc. Just add your Video and Mic to the inputs and start recording. Depending on the format you can even pause while recording.