First off, boiling doesn't sterilize as a couple of folks have said. You will not kill everything.
Adding extract late is fine and often beneficial, but be careful not to scorch it on the bottom of the pot. A small amount will not be noticeable, so don't worry if you do burn some.
I partial mash, and boil the mash with the hops and add any extract/sugars 5 minutes before I add any finishing hops (usually at 15 or 20 minutes). Once you add the extract, you will have to wait 5 to 10 minutes for the wort to get back to boiling, so I like to make sure it is back boiling before the finishing hops.
Some folks say all you need to do is get the extract to pasteurization temperature (160 F for 10 seconds). This is fine. Also, you could add it straight to the fermenter if you were sure about sanitation. Personally, I fell it needs at least 5 minutes boiling to separate break materials to help get a clearer beer.