Not possible. mIRC simple calls a function predefined by windows and sends it the necisery information. Im not sure in C\C++, but im sure its very similar to C#. You call a function MessageBox.Show("Text", "Title", MessageBoxButton.[OK|OKCancel|Cancel|YesNo|Etc], MessageBoxIcon.[Warning|Exclaimation|Etc]);

Basically, windows controls the dialog, and not mirc. The size, orientation, etc is done by windows based on what is passed. You cant change the text unless Khaled himself wrote new code (new dialog) so you can have new button text.