You can do melisma in either Synthesizer V or Ace Studio. It is easy but not necessarily quick if you want it to sound stylistically authentic. There are several ways to do it. 1) Play the notes in midi, or program them in, and have several notes for one syllable but put a dash in the lyrics (-) on all the extra notes and it will continue the syllable with the new notes. Another way is to play one note and cut it up where you want the pitch to change and manually move the pitches. A third way is to hand draw the exact pitch curve you want, sliding anywhere, not just to exact notes but anywhere in between, which is necessary in country, blues, rock, jazz or any style really. A fourth way is to sing the part with the melisma yourself and let the program copy your pitch curve. Ace Studio has the ability to do this right now, so I just ran a few seconds of George Jones into Ace Studio, and instantly, (about 20 seconds later) it gave me this:
George Jones sung by Ace Studio That was played and sung entirely by one of Ace Studios synthetic voices, and it could have been programmed by hand to do the same but instead I let the program borrow GJ's pitch curve. It not only copied the notes but also the lyrics, but it would need a bit of tweaking to get everything right. Synthesizer V will soon have, in its next update, the ability to copy pitch curves from an audio input, and since it has better sounding voices, I would wait for that if I really wanted this feature.
Instead of using George Jones, you can sing your own vocal, and slide around as much as you want. The advantage is that a girl can sing in any key and it can play it back in a man's voice and key, or vice-versa.
The first track I tried using Synthesizer V, using Natalie, was programmed by hand drawing the pitch curves, and sounds quite human to me:
Synthesizer V Natalie - Crazy