Record over existing track?

My question was not clear. Apologies. What I want to do is make an audio recording only. Then add another track on top of that audio recording.
 
This is a task for audio (post-)processing software, for example Audacity. OBS is not designed for this. If you want to add some voice-over comments to an existing video, usually video editors provide such functionality that can be used along with your other video postprocessing.
 
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