Hey Shan,
Although trial extensions look a lot like product keys, they are very different beasts. The product keys, when activated using TurboActivate, are hardware-locked -- that is, they can only be used on that physical hardware. Trial extensions are not hardware locked.
So, you can do this a few ways.
Continue to use trials / trial extensions
The first option is to hard-code some default feature values for when a customer is in "trial mode". This is the easiest option.
Use product keys for trials
If you want more flexibility (different trial levels with different features enabled) then the only real option is to use regular product keys. You can mark the product keys as "trial keys" by creating a custom license feature like "is_trial" or "trial_expires" and only set it for your trial keys.
Then you can set other trial-specific features.
Does this help?