can canadian citizens apply for EXTENSION IN USA TO STAY 6 MORE MONTHS

[om255]

My parents are Canadian citizens.They have been in the U.S. for a little more than 4 months. They can stay here legally for 6 months but I want to extend their stay for 6 more months. I need to fill in form I-539 and send it with the I-94 document (arrival\departure record) given at customs. The problem is that Canadians are not given an I-94. Is there an equivalent document that I can send the U.S. Citizenship and Immigration Services office with the I-539. If not, is there anything else I can do to extend their stay?

Thanks in advance!