Well, its not really "made", I mean, its carbonated fruit juice. To duplicate their cider's flavor is going to necessitate getting the right kinds of apples to blend to balance sweetness and flavor...ie., I doubt plain apple juice with carbonation will taste much like Martinellis. To juice your own apples though is an expensive and investment heavy proposition unless you've already got the trees and press. But if you have a good still cider you like, yes, it basically is just as easy as forcecarbing it. If you prefer Martinellis I wouldn't dilute it with water because Martinellis is quite concentrated in sweetness and flavor. But if you're drinking it more as a thirst quenching beverage, yes, I would prefer it diluted, myself.
You mention "adding fruit"...I'm not sure that is necessary or profitable. I'd find a cider you like and then forcecarbonate it. Fairly simple.
However, that's an awful lot of cider, assuming its a 5 gallon keg. Hope the missus likes it, cause there's plenty there to drink! That's why I'd lean to just going and getting a bottle of sparkling cider. I doubt it costs that much more over a comparable still cider. But I could see wanting to have it on tap just for the fun of it...