You should be able to added the extract right at the start of the full boil.
Be advised you need to do these 5 things!!!
1) Make sure the burner is OFF! Otherwise you will get burnt carmelized extract. Boil water, flame-out, add extract stir like crazy and crank her up to boil then add the hops.
2) Reduce bittering hop quantity for improved utilization.
http://www.howtobrew.com/section1/chapter5-5.html
3) Also add an extra gallon of water if you still want 5 gallons after its done boiling.
4) At 45 minutes into the boil add a wort chiller, w/o it it will take a long time to cool and you will get DMS in your beer (DMS - Cooked Vegi taste)
5) Aerate your cooled 5 gal of wort with a syphon-nozzle, paint mixer, shaking or with oxygen. Do so in the fermenter.