Extend an external application's menu

I am currently writing a plugin for a third party application.

As the plugin framework does not provide any way to access the UI I am now trying to do this manually via the WinAPI.

More specifically, I want to add a custom menu item for my plugin in the "File" menu.

My first attempt using FindWindow to retrieve the handle of the main window and the using GetMenu was not successful, as GetMenu simply returned NULL.

My next step was to use EnumChildWindows and search for a child having the text "&File" (I really don't like this approach as it makes localization quite terrible). However, I only found out the handle of the menu item, but I need the corresponding HMENU to use AppendMenu then, don't I?

Simply casting does not work and results in an "Invalid menu handle".

Is it actually possible to achieve what I am trying? How?

Thanks for your ideas in advance!

It more than likely just isn't a HMENU. Custom menu implementations are common, the one Window provides is dated and inflexible. Compare to Windows Forms' MenuStrip for example.

Of course, that blows a gaping hole in your approach.