Sorry i didn't looked at it last week. No, you don't need all those documents that they presented while applying for visa. All you need is,filled out I-539 form , copy of their I-94 and a personal letter from you mentioning why you want their visa extension...e.g., they are going to attend any special occasion, they haven't yet visit all the places here in US, medical treatment...any reasonable casue u can think of.

I have just learnt that now you can even submit I-539 online and you'll get a receipt # right away. But you still need to mail the documents like letter and copy of i-94 afterwards.

And they've raised the application fee from $200 to $300 now.

let me know if you have any questions.