You may extend your stay in B1 status by a maximum of six months provided the b1 visa extension request is due to "an unexpected or compelling humanitarian reasons" such as medical treatment or unforeseen delay in conclusion of a business matter.
You must file the b1 visa extension request using Form I-539 and prove that you have adequate financial resources to continue staying in the U.S. and that you are maintaining residency abroad.
這是我在網上找到的,看起來好象挺簡單的,I-539表格可以在移民局網站上找到。