Use Spy++ to find out what window class OE's toolbar control is.
If you find that OE uses the toolbar common control you would could get the
window handle to the toolbar and send it a TB_ADDBUTTONS message to add your
own button. You would also have to arrange to intercept the WM_NOTIFY
messages pertaining to your added button and handle them yourself. You
could do this by superclassing the parent window of the toolbar control.
Of course, this is all very ugly and could break whenever OE gets patched or
revised.
Dave