I don't think you can extend it. When I took mine there was a computer foul-up at the test center and they had to reschedule me, and there were only 2 weeks left in my "window." But they had no openings in the next week on days I could get free (getting time off during residency can be tough). The woman on the phone acted like she was doing me a gigantic favor and told me they would extend my window by another 6 weeks but this was going to be the absolute limit. And I said, "Well, it was the testing center's fault," and she said I should have scheduled it earlier in my window. I said, "Well, am I supposed to foresee that a computer problem would result in cancellation of my test date without warning?" She said, "These things happen." Then I said when I got my permit half of my window had already passed by (My window was march - July or something, and I got the permit at the end of may). She basically ignored that point.
So they don't really have a lot of customer service. I would call them, but prepare to have to pay $$.