AFAIK, the weapon/defense types are hardcoded, so unless you want to mess around in the source-code (which I do NOT advise!), you are stuck with the ones you have. The tech tree IS moddable to be less linear, but no-one seems to have done it so far. My advice: mess around with the tech editor that comes with the game: It's a great modding tool.
You CAN change the names of the weapons and defenses by modifying several files, but you are stuck with three.