Modify Internal Object Structure Using Any Design Pattern

I have a design scenario here which is quite interesting and complex. I have a Java class structure as follows,

class A
{
     class B;
     innerClass B
     {
          List<class C> listofC;
          innerClass C
          {
               String attribute1;
               String attribute2; // Their getter setters
          }
     }
}               

So I have this as an API. Now my challenge is that I need to add one more property to inner class C.  i.e attribute3 in innerClass C. I need to do this without disturbing the code in class A by extending these classes or writing a new wrapper, so I can use class C with new properties .

I hope this should be achievable through any design pattern either at runtime or design time.