I tried changing the extension name of a mkv file to avi before importing but it doesn't work. I can use VLC to convert the entire video to mp4 but it would take 3-4 hours (it's 7GB). Is it possible to convert only part of the mkv to mp4 so it could process faster? (say 00:03:00.000 to 00:04:00.000). I am on macOS. What apps may I use?