Add one option in a PHP Search Engine

It's a small work for an expert. The website is a video games database. I need the following option in the main search engine : A radio button called Pirate Games, when checked : it should include all games that have the term "#PIRATE#" in an already existing keyword field. (you don't have to create this field in the database, it's already presents). By default unchecked, all #PIRATE# games should be exluded from search. The page where to add this feature is here => [login to view URL]