not quite sure what your problem is. are you singing a tune and want a harmony based on thirds?

as Gordon says BIAB will do that in the correct key very easily. is your problem that the harmony voice sounds artificial?

use it as a guide track and sing along with the harmony line to record the harmony in your own voice. if the original melody distracts you, mute it while you record.

then delete the original harmony track and you have your voice singing the melody and the harmony.

its really quite simple using BIAB and a little thought would have got you there very easily without posting. or is this not your problem?

personally i'd do it in RealBand - much easier to play around with separate tracks in my opinion.