My guess is that you can't. I assume the workaround would be to adjust the date on your computer throughout use of the program before the expiration date in order to extend it for longer than the allotted time.
For example, if I purchased it today on June 27, 2011 and the expiration date is Dec. 27, 2011, I could use it longer as long as I kept adjusting the date on my computer and never allowing it to reach Dec. 27.