To show navigation bar or toolbar in modal view, set the value of EnsureNavigationContext property to true. This property gets or sets a value that indicates whether the presented modal view should always be wrapped in a navigation context.

Example:

protected override void ExecuteAdd(object parameter)
{
    this.NavigationService.Navigate<ItemEditorViewModel>(
        new NavigationParameter()
        {
            NavigationMode = NavigationMode.Modal,
            EnsureNavigationContext = true,
            ModalPresentationStyle = ModalPresentationStyle.FormSheet,
            CommandId = "Add"
        });
}

Hope this help.

That's cool