I think i know exactly what you're talking about.
I was able to accomplish this by modifying code from PlanetSourceCode.com in the VB Section called "Real C++ Controls" or "Real C Controls".
THis code will show you how to create a "Real C++" button on a form by passing a function the form's handle.
You can easily modify this by getting the handle to a form or control in another program.. You can do this using Spy++ or by Enumerating windows using EnumWindows() and searching for all child windows until you find a compatible form or control to create it on.
As an example, I successfully added a command button to the formatting toolbar on an AIM chat window, subclassed the buttons WndProc and had it so that one of the forms in my program was toggled visible/invisible by clicking this button.
However, I've had trouble doing this in Win2k as I BELIEVE you cannot grab the window procedure of a control that isn't in your current process thread, or something like that.
---If you need any further help, feel free to e-mail me @ mgerety@snet.net
If you do not understand the Windows API at all, I'd suggest reading up on it and doing some trial-and-error coding with it first, before trying to take on something like this.
--NipsMG