You could look at my program, which attempts to add chords and/or appropriate harmony notes to a tune. It can be downloaded from my site www.greenhedges.com
If you select 'Add 3rds' or 'Add 5ths' then the third or fifth will be added to each note in the tune.
You can add BOTH if you like. This is MOSTLY, but not always, harmonious.
The new version, 2.0.2, available soon (hopefully after Easter), will let you add 4ths above or below the melody. 4ths above give an oriental sound, and 4ths below are a very early form of harmony. (Actually, the same notes as a 5th above, but then lowered by a whole octave)
(PS The new version, 2.0.2, will also play a single bar, or will let you play the notes one at a time, forwards OR BACKWARDS. It also handles abc lyrics)