Not this year, but I did that my first NaNo - started at Chapter 6. I wrote 50,000 *not including* the first five chapters (and therefore ended with a total of about 70,000 words.)

It's not technically within the rules, but as Dawn mentioned, you should check out the NaNo Rebels. In the spirit of NaNo, if you write 50,000 words of anything within 30 days, you've done a good thing.

Regards,
Michelle