It depends on if they expect you to add more sugar or not. If they think you'll use it as is, adding unhopped extract will make it seem less bitter than it already is. Still, I would probably leave out any bittering addition this time around and see how it turns out.
Hmmm, I didn't consider that. I can't tell from the recipe if it assumes you will add additional sugar (following the directions on the can) or if the dry malt extract and grains in this recipe makes up for that. My guess is that the grains and dry malt extract replace the sugar you would use if you were just brewing the kit as is. If I added more sugar, would that just increase the gravity and therefore the alcohol content, or would that make the beer too sweet?
When I said sugar, I meant extract sugar as opposed to table sugar / sucrose. Adding something highly fermentable like sucrose or glucose will increase the alcohol and reduce the body because they attenuate fully. They will not increase the sweetness of the beer if you're adding reasonable amounts. Adding malt sugar in the form of extract will tend to increase the body and decrease the bitterness because the extract it is not
Did you come up with this recipe on your own, or is it one that someone has done before? If it's someone else's recipe I would just follow it and see what happens. If you came up with it on your own . . . well, you're not adding so much to it in the way of grains/extract that I think you really need to worry about the bitterness, so just follow it and see what happens.
Let me sum up: don't worry about it, brew it as you have it written and let us know how it goes.