Could you copy the melody to the soloist track, and then transpose the Soloist track by a third?
(Not every note would necessarily sound correct, but it may be a starting point)
Yes. Not every note is right. Because the third pitch needs a limit, that is, it must be in the correct KEY, and a SCALE must be set
I just searched, and there is no similar software in the world at present. There are many people who want to set the SCALE according to a melody, and then generate a third.
This makes it easy to sing the harmony part.