This is called "Late Extract Additions" and some people prefer the results over boiling the entire amount of extracts for the entire time. It is supposed to help keep the beer from darkening too much. Extracts have a tendency to be darker than All Grain.
It will also change the flavor a small bit. And it will allow the hops to isomerize a bit more, so you may have to adjust the hops.
I would add it in during the last 15 minutes. This won't make a real impact on teh flavor of the beer, yet still provide for more sanitizing, and give you a few more minutes to prepare the chill if necessary.
Sometimes the sun shines in, and I close the blinds, cause I can't stand to see the light of day. I have no use for pain, but I don't mind the rain, cause I can stay inside and hear the sound.
Today I look right down, right down to the ground, there's nothing in between to break by fall. I'm blue for you, and I don't know what to do.