I use "Center across selection" and almost never "Merge cells", which is
generally a bad choice (across columns anyway).

I wish to add an icon or otherwsie modify the "Merge Cells" icon through the
toolbars "customization" process. but unfortunately I cannot find "center
across selection" amonst the "commands- format"choices.

Which means I have to do my "center cross selctiion) always via the menu....

How do I add / modify an icon in my "Formatting" toolbar?