title: Extend existing FormBuilder.group after initialization directly instead of adding FormControl one by one
body: <p>I am using reactive from in Angular 7.
  I am creating child component for user profile edit.
  I have multiple role wise user pages.
  In all these pages user profile fields are common. 
  So I have created user profile component in which I have an input parameter FormGroup:</p>
  
  <pre><code>constructor(private fb: FormBuilder) {
      this.myForm = this.fb.group({
        test: ['', Validators.required]
      });
  }
  
  &lt;user-profile [formGroup]="myForm" [submitted]="submitted"&gt;&lt;/user-profile&gt;
  </code></pre>
  
  <p>Now I want to extend input FromGroup to add user profile fields. For ex, I'd like to extend/add incoming/input FormGroup like below from my child component:</p>
  
  <pre><code>.group({
    username: ["", Validators.required],
    email:["", Validators.required],
  });
  </code></pre>
  
  <p>Can anyone suggest me how  can i do that?</p>
  
  <p>I have more then 20 fields in my child component. So i don't want to add FormControl one by one. Is there any way to extend JSON like  below in existing form-group </p>
  
  <pre><code>{
        username: ["", Validators.required],
        email:["", Validators.required],
  }
  </code></pre>