How to extend mpz_class in c++

Question

I want to create my own class that is similar to mpz_class except has a few extra methods. I have however heard it is not a good idea to extend std classes. Does this advice apply here or can I safely do:

class MyBigInt : public mpz_class {
public:
    long double specialMethod() {
        //do something that returns a long double
    }
}

The reason I am interested in extending is I still want the ability to do

mpz_class a,b,c;
a=5;
b=6;
c=a+b;

so if extending is a bad idea I need to some how figure out how to make the above work with my own class.