It is not clear to me what the requirement is?

If you need to find what chords are BEING played in a MIDI file, yes, you need my program Find MIDI Chords. (This was previously a commercial program distributed by a major supplier of professional MIDI files.)

If you have a melody line only, and wish to ADD chords to it, then my program HARMONY can import it, add chords, and also output as a MIDI.

You can also edit and transpose before outputting.

As John says, they are both available at my web site http://www.greenhedges.com