There's a pretty fundamental "design pattern" which will do: the
interface. Declare a plugin interface, which 3rd parties can write
implementations of.
i.e. you have something like:

public interface Plugin {
  public int connectToChicken(int legs);

  public int pollShatner(long kidneys);

  public void close();
}

3rd parties then provide classes which implement this interface.

Also see 'delegation', and perhaps 'strategy pattern'. (Search the web
for these terms.)