Preferably you would look at implementing them as tags rather than making method calls, but you could also consider just moving the functionality out to a separate helper class.
Re-iterating the opinion of the other experienced developers: this is not an appropirate way to extend the capabilities of a JSP.
Not only is it pointless; interfaces are for allowing external code to deal with an object instance in a specific manner, and there is no external access to a JSP. But there are many supported means to extend JSP behavior: custom actions, EL functions and so on.