Is there a way in Flutter to add a NavBar to a route outside of the Shell Routes?

Question

In Flutter using the go_router library I am adding pages A, B and C, which exist in a BottomNavBar navigation. They are all encapsulated in a ShellRoute with a BottomNavBar and work like a charm.

My question is how can I add this BottomNavBar to a new screen D, which is not in the Shell list?

Just by adding the D route to the ShellRoute, I get the NavBar, but I cannot navigate to A (the first index), because it's selected by default.

I am using Flutter 3.7.5 with go_router 6.2.0, which I pass to a MaterialApp.router.

ShellRoute:

ShellRoute(
  navigatorKey: _shellNavigatorKey,
  builder: (context, state, child) {
    return ScaffoldWithBottomNavBar(child: child);
  },
  routes: [
    GoRoute(
      path: '/a',
      builder: (context, state) => const A(),
    ),
    GoRoute(
      path: '/b',
      builder: (context, state) => const B(),
    ),
    GoRoute(
      path: '/c',
      builder: (context, state) => const C(),
    ),
    GoRoute(
      path: '/d',
      builder: (context, state) => const D(),
    ),
]
),

Scaffolded BottomNavBar:

class _ScaffoldWithBottomNavBarState extends State<ScaffoldWithBottomNavBar> {
  int get _currentIndex => _locationToTabIndex(GoRouter.of(context).location);

  int _locationToTabIndex(String location) {
    final index = tabs.indexWhere((t) => location.startsWith(t.route));
    // if index not found (-1), return 0
    return index < 0 ? 0 : index;
  }

  // callback used to navigate to the desired tab
  void _onItemTapped(BuildContext context, int tabIndex) {
    // The problem occurs here - on screen D 
    // _currentIndex == A and we don't navigate to A
    if (tabIndex != _currentIndex) {
      context.go(tabs[tabIndex].route);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: widget.child,
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        items: tabs,
        onTap: (index) => _onItemTapped(context, index),
      ),
    );
  }
}

Update

After doing a research and brainstorming, my conclusion is that screens that are not in the NavBar items shouldn't have a NavBar in the first place. They act like a modal and have a back button instead.