That's the whole glory of the trial period, it's a way of programer's to let consumer's try out their product before they buy the full version. i hate it when they use to give you 30 to 60 days, and now some are only 15 days... bummer.
their is no way of extending the trial period, unless you find a crack for the software and open it to the full version, but the problem with that, is that if you update the software it will detect that it's un-registered the correct way.