If I understand correctly, this already exists in mIRC 6.14 and above. Upgrade to the latest mIRC
:"03/03/2004 - mIRC v6.14
64.The switchbar now displays scroll buttons when it is vertical and unable to fully show all windows. The mouse wheel can be used to scroll through the window list."
Just an FYI, the row of tab buttons that display the network names and channel names is called the "switchbar".
Hope this has been informative!