What you're trying to do is add new items with new functionality that isn't present in the existing game. This will entail getting your hands around the game's entrails, figuring out how it all works, and making your own changes. In short, this is a very advanced hack you're talking about, and you appear to be a newcomer to the field.
My recommendation? Put this ambitious project of yours on the back burner and aim for more low-hanging fruit. You don't have to give up on hacking MGGB, but you should consider trying something a bit more in your skill range, like text or graphics hacking. We have all the documentation and tools you could ask for on the left.