I'm not sure I fully understand what you're asking, but.. If you know the window handle of the button, you can send it the BM_CLICK message -- "An application sends a BM_CLICK message to simulate the user clicking a button. This message causes the button to receive the WM_LBUTTONDOWN and WM_LBUTTONUP messages, and the button's parent window to receive a BN_CLICKED notification message."

hxxp://msdn.microsoft.com/library/default.asp?url=/library/en-us/shellcc/platform/commctls/buttons/buttonreference/buttonmessages/bm_click.asp