Easy not really, unless you know what you are doing, and since you are asking...
Subclass the toolbar window handle using SetWindowLong(hwnd, GWL_WNDPROC, ..). In the subclassed procedure, create a switch to handle window messages, always passing the messages to the previous windows procedure after processing them first.
You will need to intercept and stop mIRC from receive messages such as WM_SIZE and WM_MOVE. Since mIRC isn't getting these messages anymore, you will need to (re)size the window in your subclassed procedure (look at SetRect() and GetWindowRect())
You could also wait until I have time to finish my DLL that will do this and much more (like completely DLL based treeview switchbar)...
