Let say I have some third-party program installed on my computer with Windows 7 OS.
When I run it, I see the following:

I don’t know anything about the program innards and all I want is to override it’s GUI to display some extra information (red items on the next picture):

I guess it is possible, but I have no idea which way to dig.
You can write a Windows Hook.
I have made this a couple of times in the past to modify the behaviour of same third party applications. But that was a lot of years ago (Windows NT and Windows 95).