Há uma boa explicação das questões envolvidas no site de consultoria jurídica de renome nolo.com: Como obter um visto nos EUA (B-1) para fazer o trabalho voluntário .
When the Customs and Border Protection officer greets you and reviews your passport and visa, you will receive a stamp in your passport that notes the date you arrived, your visa class (B-1 visitor), and how long you may remain.
With a B-1 visa, you should receive a period of stay of up to one year. If you receive a shorter period, or if your volunteer program will last longer than the authorized stay noted in your passport, you can later submit an application later (Form I-539), with filing fee and supporting documents, to U.S. Citizenship and Immigration Services to extend your stay. [It goes on to explain how to do this.]