I want to take an existing enum and add more elements to it as follows:
enum A {a,b,c}
enum B extends A {d}
/*B is {a,b,c,d}*/
Is this possible in Java?
No, you can't do this in Java. Aside from anything else, d would then presumably be an instance of A (given the normal idea of "extends"), but users who only knew about A wouldn't know about it - which defeats the point of an enum being a well-known set of values.
If you could tell us more about how you want to use this, we could potentially suggest alternative solutions.