could put the voice thru a vocoder like Digitech's "Talker" (or others), and use a keyboard signal input for the other sound source, and play the harmonies along with the singer as she sings, and it comes out in a harmony tied to the chords you play on the keyboard.
Other than that, i think Alesis[?] have released back in the 80's, a vocal harmonizer unit, which had various mode presets, in which it would harmonise along according to a scale that u set. It was ages ago tho that i used it.