Is there a way to increase the alcoholic content of a beer?
More grains or extract. Or sugar. Why may I ask? Adding too much sugar will not make great beer.
Sugar, in some form. Or, add liquor.
Table sugar will do the trick but it will also dry out the beer.
A bigger malt bill = more sugar
You could add more DME or LME to boost without drying.
There's always this product. http://www.austinhomebrew.com/product_info.php?products_id=10137
That's the Brewvint Alcohol Booster, though I have never tried it.