I am NOT the most experienced Sonar/Melodyne user on this forum, but I think I can give you some small bit of guidance anyway...
Another tool that also comes with X3 might be far more appropriate for what you have in mind if you're a keyboard player. It's called
Strum Acoustic Session.
What you describe could likely be done with Melodyne Editor. I'm considering it myself and I'm about to start asking questions here and on the Celemony forum to determine whether or not it's something I need-need or just think I need... if you know what I mean.
My understanding, so far, is that yes, you could take a single sampled chord and manipulate it with Melodyne to patch together a song. BUT (and you'll note the all-caps BIG but) speaking from the POV of a guitar player, I doubt it'll bear much resemblance to a real guitar. One of the things experienced guitar players can do is listen to chords played in the first position of an acoustic guitar hear which chord shapes are being played. If every shape is a C shape, but pushed up or down, it'll just sound weird. Even piano samples, where one would half-expect your method to work best, would sound off because each note on a piano is sampled from the actual string that plays that note on the piano. It's possible that only a guitar player would notice the difference (as a keyboard player might notice in regards to a piano) but I think anyone who listens to acoustic guitar music much at all would know something wasn't quite right.
So, bottom line, in your position, I'd still upgrade to X3, but not necessarily for the Melodyne Essential plugin and Editor upgrade (and they've extended that sweet half-off deal for the upgrade from Essential to Editor which is why I'm looking for a reason to buy it myself).
For the purpose you've described, I'd upgrade to X3 to get Strum. And the version of Strum you get with X3 isn't a striped-down version, either. It's the real deal.
And before you ask, no, I haven't actually used Strum. Being a guitarist, I prefer to stumble through my own guitar parts and I haven't yet found a need to bolster a guitar part with Strum, although I won't discount that as a possibility yet.
At this point, I don't really know enough about Melodyne Editor to say one way or the other if it's so much different from Essential to make it worth having, but as I said, I'll be researching that very question over the next couple of weeks. I hope you'll come and ask some of your own questions if I can get a thread going on all this.
Hope that helps.