Getting the alcohol content right

For my next experiment with cider I plan to ferment three gallons of cider. When its done fermenting (thinking 6 months or so) I'll stop the fermentation and mix it with two gallons of unfermented cider and force carbonate. That way I can return a little bit of that apple sweetness. But in doing so I realize that my alcohol content will be low as the cider will be diluted by two gallons of unfermented cider.

So, how can I raise up the alcohol content so that when it is blended the alcohol content is correct? I'm figuring that I'll have to add some corn sugar but I'm not sure how to calculate how much to add.

have you thought about throwing in some apple concentrate at the end to boost the flavor without changing the overall volume, and therefeore the alcohol content too much?

Klainmeister has the right idea. A can or two of concentrate is all it will take.

Unless you use sugar to boost the ABV, apple cider will ferment in a week. It does need to age 3-6 months.

A pound of sugar = 46 points. So, a pound in five gallons adds 0.009 to the gravity and about 1.2% ABV. This is one case where using table sugar isn't a problem, unless you don't like cidery tasting cider.