Unfortunately the toolbar on the Macintosh is not customizable for specific tools that way. You are limited to the tools shown in the Customize Toolbar dialog.
Is this true even for custom addins? I have several addins, which are each custom toolbars calling various scripts. One is just a shell script to call the concatenate function, but to modify the GUI to widen the list boxes (my file names are long).
Edit: To clarify, I use windows. But I was curious if the same could be done for mac.