Extending Protobuf with my own methods

How should I add methods a Protobuf message?

Suppose I have in my .proto file:

package proto;
message Person {
  required string name = 1;
  required int32 id = 2;
  optional string email = 3;    
}

and I want to add a method, say, string concatenateNameEmail() to the message.

What I do right now is I create my own C++ class like this:

class Person : public proto::Person
{
public:
  Person( proto::Person const & person_ )
  : proto::Person(person_)
  {}

  string concateNateNameEmail()
  {
   ...
  }
};

So the downside is I need to call proto::Person copy constructor. Is there a more elegant solution than this?