Dry hopping won't increase the bitterness (though you probably know that). You could get increased bitterness a few ways. The simplest is to use isomerized alpha acid extract, e.g. http://morebeer.com/products/isohop-...ract-1-oz.html
. That's just the bittering components of hops, in pure form, and a little goes a long way. It's expensive though, and you may not need it again. So more rough and ready solutions include making a small amount of bitter hop tea on your own. Take some hops, boil in water for 90 minutes, and after cooling, add that at bottling. You can experiment with proportions to get it to the desired bitterness. You could also make a small amount of much more bitter beer, and blend the two beers (a more complicated version of the hop approach).