Could I use this section for a personal statement?
Yes, but it’s in your best interest to only includes things that will enhance your application or better explain things written elsewhere in the app. Don’t include an essay that you wrote for another school just because it’s well written or because you’re afraid to leave this section blank.
@rothstem My common app personal statement is covering an event that is listed on my MIT application. It is well written and will give me a chance to talk about this event without word restrictions. I don’t want to create an additional work for a reader of my application and don’t know if this is appropriate.
I would include it as long as it’s really well written. The way I see it, it’s your basically your last chance to make an impression on your reader, so I’d use the space and use it wisely. Even if they barely skim over it, it’ll add another dimension to your application. Since it’s the common app essay, I assume that it’s less than 650 words so it’s not like you would be adding hours of work on their part.
There’s a variety of ways to use this space. For example- I’m going to briefly elaborate on my relationship with my sister. We’re in the same grade, she was adopted, we’re polar opposites personality-wise, and we’re in 5 classes together this year. I’m going to show that I have come to appreciate differences in people through my experience growing up with her.
So, I think that using the space in almost any way that makes sense to you but will undoubtedly paint you in a positive light is the best way to go. Good luck! 
There’s a lot to cover already for the adcom. If they sense it’s from a common app prompt, I don’t think they might be very happy.