I've done one app previously in Sencha Touch so I'm still new to the framework, but now I am working on building an app in Designer and I'm somewhat confused on some of the workflow habits.
I'm wondering where the appropriate place would be to add arbitrary child components to widgets laid out in the designer?
Specifically, I have created a Navigation View and I wanted to add a selection widget to the nav bar. But the way Designer keeps a lot of implementation sections as read-only, I can't define the widget in the normal fashion. One way I have gotten it to work so far is to add the selectfield to the nav bar in the init of one of my controllers. But I have to do it inside of a callback that returns later once everything has been instantiated.

What is the expected workflow for this situation? Is there a common place to add these child widgets through Designer?