If it was me, I would do it in a dirty way, just to see results quickly.
1. Use Handbrake program to convert mkv to something that iMovie and Garageband can work with.
2. Drop that movie file to Garageband to have a track of original audio with video window.
3. Playback original track and record dubbing on a separate track in Garageband.
4. Mix it in Garageband with your dubb louder and bounce the audio.
5. Drop into iMovie both - that same movie file that Handbrake produced, and bounced audio from Garageband.
6. Replace audio in iMovie and export the file.
Mkv is a pretty sweet container, it can handle multiple audio tracks. You could use something on windows with paid software to just add recorded file to mkv. My way is bit more hassle-involved but its free.