I've figured out how to add the field using an override. It would still be good to find a way that doesn't rely on this though.
Code:
Ext.define('App.view.override.MainContainer', {
override: 'App.view.MainContainer',
constructor: function (config) {
var navBar;
this.callParent(arguments);
navBar = this.getNavigationBar();
navBar.add({
xtype: 'selectfield',
options: [
{
text: 'Dashboard',
value: 'Dashboard'
},
{
text: 'Clients',
value: 'Clients'
},
{
text: 'Advanced Search',
value: 'AdvancedSearch'
}
]
});
}
});