If it's possible at all, you're probably going to have to use pInvoke and one or more Windows API functions to do it. Why do you want to add tooltips to those buttons?
EDIT (after comment) ==============
You might want to find a way to handle the
) for the titlebar, then. I'm not sure how you'd go about doing that in .Net, but google may come up with something.
Overriding WndProc would look like this:
protected Override void WndProc(System.Windows.Forms.Message msg)
case WM_NCMOUSEHOVER :