You are expressly allowed and encouraged to add specializations to namespace std*. The correct (and basically only) way to add a hash function is this:

namespace std {
  template <> struct hash<Foo>
  {
    size_t operator()(const Foo & x) const
    {
      /* your code here, e.g. "return hash<int>()(x.value);" */
    }
  };
}

(Other popular specializations that you might consider supporting are std::less, std::equal_to and std::swap.)

*) as long as one of the involved types is user-defined, I suppose.