Third-Party Animations for Keynote?

Hello,

I'm using Keynote to create some simple presentations for my students that I will use in class and then make available on the web as short movies. I would like to add some different animations to Keynote than the ones that come with the app. For example, I would like use an eraser to remove some text, or at least add a build-out effect that would show words being erased. Does anyone know if this is possible to do in Keynote, and if so, how? If someone has a lead on where to find those animations, please let me know.

Thanks!