// A *self-contained* demonstration of the problem follows...
// ===== file Foo.ts =====
export default class Foo {
  hello() { 
    console.log('Foo'); 
  }
}
// ===== file Bar.ts =====
import Foo from './Foo';
declare module './Foo' {
  interface Foo {
    add(x, y);
  }
}
// ERROR: 'add' does not exist in type Foo.
Foo.prototype.add = function (x, y) { return x + y; };
let f = new Foo();
f.hello();
f.add(3, 4);   // ERROR: 'add' does not exist in type Foo.