In Typescript, create extension method for class that from library

I want to add some method to existed class that come from library.

Known some basic type like String, Element can extension via interface and add it's prototype.

interface String {
 extFunc():Function;
}

String.prototype.extFunc = function () {
  //blabla
};

"a".extFunc();

I also find the way to add extension method of Observable<T>.

declare module 'rxjs/Observable' {
  interface Observable<T> {
    foo: String;
  }
}
Observable.prototype.foo= "bar";
console.log(Observable.of("a").foo);

But when i try to do same thing to NavController(an lib from ionic), it will override entire NavController to the interface that i declare.

declare module 'ionic-angular' {
  interface NavController {
    replace(page: Page): Promise<any>;
  }
}
navCtrl.replace(...); //ok
navCtrl.push(...); //original function => "not exist"

Does anyone know what is best way to add extension method to those class? thanks~

Edit: There is actual code

import {Component} from '@angular/core';
import {NavController} from 'ionic-angular';

import {GuidePage} from '../guide/guide.component';

@Component({
  selector: 'page-home',
  templateUrl: 'home.component.html'
})
export class HomePage {
  pageName = "home";

  // navCtrl is come from angular2's Dependency Injection,
  // it may not suitable to extend NavController
  constructor(public navCtrl: NavController) {}

  gotoGuide(): void {
    //wanna replace this
    this.navCtrl.push(GuidePage).then(() => {
      let index = this.navCtrl.getActive().index;
      this.navCtrl.remove(index - 1);
    });

    //with this custom extension method 
    //this.navCtrl.replace(GuidePage);
  }
}