BigBry68 said:
Does it matter that I already pitched the yeast?
If you used a kit, or measured your components accurately, and if you added the correct amount of water, the original gravity WILL be correct. Your wort may not have been thoroughly mixed when you took your OG sample. It's very difficult to get the wort mixed well enough that a random sample is accurate.
By this time the yeast has had time to ferment a large portion of the sugar in the beer, so you get to choose (1) you can bet that your original gravity reading was correct and therefore the beer needs the additional extract to meet your target, OR (2) bey your original gravity was erroneously low and adding extract will give you a stronger, sweeter beer, OR (3) you can let it finish as it is and take a chance on getting a very weak beer. If it were me, I'd add the extract and count on getting either the predicted beer or a stronger beer - seems like the best combination of possible outcomes.
I'm a fan of cool fermentation, at or a couple of degrees below the yeast manufacturer's published range. Please note that's the yeast manufacturer's range, not the kit packager's published range. Kit instructions vary from minimal/inadequate to bad/wrong. Google your yeast to see what the manufacturer says. If you have been fermenting very cool, your yeast may not have fermented an imperfectly mixed beer below your original measurement. If you take a gravity reading and it is close to your original reading, your original gravity was probably correct.
At this point, the beer will be thoroughly mixed, courtesy of the yeast activity. It won't matter that you've pitched the yeast. If you decide to add extract, boil the extract briefly, cool it and add it. You shouldn't need to stir it; the yeast activity will mix it adequately. Stirring might increase dissolved oxygen which might harm the beer at this point (Or, it might not harm the beer. You can get plenty of conflicting opinions on that subject - and most any other subject you bring up.)