Force a friend outside of class declaration?

[babaliaris]

Is it possible to force a class to have a friend class or friend function outside of this class declaration?

For example

class MyClass
{
 . . .

private:
    int m_id = 0;

};


//Somewhere else
force MyClass to have friend int AccessPrivateSruff(const MyClass& obj);


int AccessPrivateSruff(const MyClass& obj)
{
    return obj.m_id;
}

The reason I want to do that, is because I'm writting a unit test framework and
I want somehow to add my unit test class as a friend to the classes that I'm testing
but I dont want to change the declarations of the actual classes that I'm testing.

It does not make sense my actual application classes to have friends of the testing framework,
since the testing framework is not part of the application.