i'm assuming you are fly-sparging? i don't see any reason why this wouldn't work. the first runnings will still be the strongest, no matter what, but the continued runnings will still be suitable to make a decent beer. adding extract to that shouldn't be a problem.
i batch sparge, so the first and second runnings are for the main beer. usually when i do a small beer with the 3rd runnings i end up with a very small (2-3%) beer that has a little tannin taste. balance that with some hoppy aroma and flavor and it's good for what i like to call "free beer"...which can be consumed in a matter of days. i don't think it would be as good if i added extract to that, tho.