manticle is spot on. Might be darker and a little more bitter. But it will be drinkable still - just won't be the light pils you expected.
General approach with boiling is add any pre-hopped cans at either flame-out (at the end of the boil when you take the pot off the heat) or simply add the can to the fermenter when you strain the boiled liquid to it. If adding unhoped malts (in liquid or dry form) you can add some at the beginning of the boil and then the balance at the end. When I do an extract recipe and boil for 30 or 40 mins, I like to throw 1/3 of my dry malt in at the start - it seems to give it more body and flavour.
Cheers,
Pete