Now I'm not entirely sure but as it sounds, you have an interface called WalkableInterface?
If so, you want your classes that are supposed to be a WalkableInterface to implement the interface.
I.e class PvPZone implements WalkableInterface
If this is not the case, and there is a method to determine if something is a WalkableInterface you might want to call that method somewhere adding your new interface to a list of WalkableInterfaces.
Can you send a link of the code you're referring to?
Or do you need two interfaces at the same time to overlap each other?