Are you using the plugins to extend the functionality of classes that are embedded in the main application? Thats the only thing I can think of that you might be trying to do if the Interface method isn't appropriate. For example, given the vehicle scenario.
The classes Car, Bike and Bus are all internal classes in the application and have a few methods each. The application also includes a plugin system that passes classes to the plugins (as cast by an interface). After deploying the application you decide that each class should have included a method / property for description.
In order to add this method without altering the original application you distribute a plug-in dll. This plugin recieves the object decides what it is and displays a message containing the description depending on the type. This would not help you add a class (for example aeroplane) but would allow you to make some basic additions.
Now if this is what your attempting I'm sure there would be a neater solution by making the classes Car, Bike and Bus plugins that implement an interface which is itself seperate from the main application.
If that isn't what your attempting then i'm stumped.