According to Path2usa, B1 and B2 visa holders can extend their stay if:
You have a valid legitimate reason to request for visa extension, under the visa category.
You were lawfully admitted into the United States with a non-immigrant visa
Your non-immigrant US visa status remained valid
You have not committed any crimes that make you ineligible for a visa
You have not violated the conditions of your admission to the USA
Your passport is valid and will remain valid for the duration of your stay.
You have definite plans to leave the US at the end of the proposed visa extension period.
Proper evidence for financial support is provided.
What Documents Do You Need to Complete for a US Visa Extension?
You’ll need to:
Fill out and complete form I-539. You can do this online, by the mail, or in person at a USCIS office
If you file online, you’ll need to understand how to submit attachments of the document you’ll need
File an application (letter) stating the reasons for requesting a visa extension
Provide proof you can support yourself financially during the extension – such as bank account statements and employer statements
Supply a copy of your return airline or shipping tickets
Pay the filing fees ($370) and any biometric fee ($85). Generally, your spouse and children are included in these fees
Provide a copy (not the original) of your I-94 for each applicant
Be sure to read the extension checklist and to follow the sample cover letter that the USCIS provides.
When Should the Visa Extension Request be field?
The USCIC recommends that you request an extension at least 45 days before your visa expiration date ends. Generally, USCIS must receive your completed application and fees by the day your authorized stay ends.
What Happens After you File the Extension Request?
The USCIS will send you a receipt with a 13 digit case number. “The approximate processing time will be indicated on the receipt.” You may be given a biometrics appointment so you and anyone else on the application can be fingerprinted and photographed
If you file your extension request on time, you may be permitted to stay in the US for up to an additional 240 days depending on the reasons behind your request and other factors.
You can check the status of your request online through the receipt case number.
If the visa extension is approved, you will be given a replacement I-94 with the new expiration/departure date. You should make copies of this I-94 so you’ll have them for subsequent trips to the United States.
If the visa extension request is denied, you’ll receive written notice of the denial. You will need to leave the US immediately.
Any additional tips?
Be sure your travel medical insurance is current and covers the extended stay.
Generally, you should be prepared to leave so you’re ready if the visa extension is denied. If the extension request is granted, you can follow through with your alternate plans.
I have a B1-B2 visa and I want to extend my stay, Should I apply for a Visa extension OR just go to Canada Or Mexico and re-enter, will I get a new I-94 within 6 months?
“B1 and B2 visas are usually issued for a term of 10 years. Each visit may last up to six months, although some categories of visitors may apply to extend their visit for an additional 6 months. During your visit to the U.S., you may visit Canada, Mexico, or the Caribbean islands (not Cuba)for up to 30 days and re-enter the U.S. as long as you re-enter within the period noted on Form I – 94 which you received when you first entered.”
How Long you can Stay after Applying for a Visa Extension?
“If USICS receive your application before your status expires (or, in exceptional cases, we excuse filing after your status expires due to circumstances beyond your control), and if you have not violated the terms of your status and meet the basic eligibility requirements, then you may continue your previously approved activities in the U.S.( including previously authorized work, for a period of up to 240 days), until we make a decision on your application or until the reason for your requested extension has been accomplished- whichever comes first.”
What if I file on time for a Visa Extension, but I leave America before USCIS makes a decision on my application?
“If you leave the US before a decision is made on your application to extend and you plan to return to the U.S. in the future, please keep a copy of your application plus the receipt notice to show to the Immigration Inspector on your return travel to the U.S. Otherwise, you may be denied entry for overstaying on your last visit.”