Append/replace menu items from within xaml and C#, based on user action

Question

I have an app that allows the user to manipulate different types of data (images, text, whatever). When the user is working with one of these custom User Controls, I want to append a new drop down menu to the main menu. For example, in my ImageEditor User Control, I have the following xaml:

<UserControl.CommandBindings>
   <CommandBinding Command="cmds:ImageCommands.GrayscaleImage"
                  Executed="OnImageCommandGrayscaleImage"
                CanExecute="OnImageCommandIsGrayscaleImageAllowed" />
</UserControl.CommandBindings>
<UserControl.Resources>
   <MenuItem Header="_Workspace" x:Key="WorkspaceMenu">
      <MenuItem Command="cmds:ImageCommands.GrayscaleImage" />
   </MenuItem>
</UserControl.Resources>

What do I have to do to get this resource added to my main menu when this control takes focus?

(I know I can build all this in C#, but I'm largely doing this project to learn WPF and xaml. So, if there's a way to make this happen entirely within xaml, or by defining the UI in the xaml and then loading it in code, that would make me happiest.)

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After some research, I've discovered the following almost works, but not quite:

MenuItem myMenu = (MenuItem)FindResource("WorkspaceMenu");
mainMenu.Items.Add(myMenu);

The menu from the resource is added to the main menu, but none of the command bindings are correct. (e.g. dropping down the menu from my example doesn't trigger OnImageCommandIsGrayscaleImageAllowed.)

---

More research: The ImageCommands.GrayscaleImage command gets triggered on the main menu's owner, NOT on this custom User Control. By changing the code to this:

MenuItem myMenu = (MenuItem)FindResource("WorkspaceMenu");
myMenu.CommandBindings.Add(new CommandBinding(
                                  ImageCommands.GrayscaleImage,
                                  OnImageCommandGrayscaleImage,
                                  OnImageCommandIsGrayscaleImageAllowed));
mainMenu.Items.Add(myMenu);

It all Just Works. However, I would really like to know why I have to create that CommandBinding in the code behind file, instead of the xaml.