Here is a macro that joins selected files in the Finder with MP4Box:
Join files with MP4Box.kmmacros (5.0 KB)
The joined file will be saved to the directory of the last item in the Finder selection. You can set the name of the joined file in the first (green) action.
Make sure that KM’s ENV_PATH variable contains the path where your MP4Box is located. (Usually /usr/local/bin if installed via Homebrew.)