I think that there are a couple of questions that need to be cleared up before a satisfactory answer is devised here-even if there turns out to be a software program that can write a harmony line--

First, what is the harmony part for? Is it a voice part that is desired, or is it an instrumental harmony?

If you want a computer program to create a harmony part, you have to remember that there are different kinds of harmony parts that go with different types of music--

If you find a program that will write a harmony part, it is still necessary to figure out whether it is an appropriate part for the music that you want to perform--

Folk music genres each tend to have their own fairly specific rules for harmonizing and accompanying, and even when they happen to follow the rules of classical harmony(which they don't always do) classical rules would allow a lot of notes and parts that would sound OK, but wouldn't sound exactly right--

I am a composer (more or less) and learned to write harmony parts by the theory rules (or actually by a lot of different theory rules) before I got so that I could just sing or play a harmony part for a given song (I really sort of cheat on this, because I have figured out musical rules for the parts, and I figure it out as I go, rather than just feeling the part)--

Rather that trying to work with a computer program at all it would be much easier to just listen to the music and ask persistant and pointed questions of everyone who plays and sings(like here in the forum) so that you can figure out how to do it yourself--

There are two reasons--first, assuming that there is a progam that does what you want, you would still have to understand enough about harmony to know if the part that the program wrote was right, and if (actually when) it wasn't, how to change it so that it was.

Second, writing a harmony part is like writing a sentence--it is the expression of a human idea, you don't count on the word processor to create the sentence, you have to have know what you want to say first-only then you can use the computer to cut, paste, and otherwise edit--

there was a discussion a little while back where people talked about how they figured out their harmony parts, and there were some great tips--only thing is that I don't remember the thread name--