"extending" existed class

[novice2]

Hello.
I have existed class (for example Stream in phobos), and i need add
new method to it. In the past (if i correctly remember), i was able
to do it by defining function, that accept instance of class as
first argument, and then use it function like instance.function()

Now (DMD 1.70) i can't compile this code:

//////////////////////////////
class A
{
  int method1() { return 1; }
}

int method2(A a)
{
  return 2;
}

void main()
{
   A a = new A;
   int i = a.method1();
   int k = a.method2();
}
//////////////////////////////

Compiler say
1.d(20): no property 'method2' for type '1.A'

Can anyone say, what i missed, or the better way to "extend"
existed class without declare new class based on existed?
Thanks.