This is not a good topic for the OBS Studio support forum, since adding audio after the recording is a task for some kind of postprocessing software. Search for "Video Editor" in your favorite web search machine. There is a vast field of video postprocessing software from free to many $1000. OBS itself doesn't have a function to merge audio to existing video files.
The workflow in general is this:
- record your video with OBS, with or without care for audio
- use other software to record additional audio, or use OBS to record a black video but with the audio sources you intend to use
- use a video editor to extract the video parts of your video recordings you want in your final video
- use a video editor to extract the audio parts of your video or audio recordings you want in your final video
- use a video editor to merge the video and audio parts to your final video
This is a common workflow used for every non-trivial video. Often, you invest more time in postprocessing than in recording the raw stuff.