Require a code to purchase a specific product

Hello there,

I would like to know if there is a way, maybe a plugin or a script, to require a special code (a sort of license key issued previously) in order to purchase a specific product, which would be an extension of the original product.

The thing is, I do not want registered users on my site, so everyone is a visitor actually. Upon entering a valid code, perhaps in the checkout page, the site will let you purchase the item. The best solution would be the opportunity to add one of the available valid codes, since the special code should be unique.

Thank you in advance,
Tom