Tool (and menu) bar items have their own interal click event
that calls the function you specify in your custom command
bar item's OnAction property.

The built-in command bar items call their own internal
procedure.  You can change the OnAction property of the
built-in buttons, but it would be a bad thing because it
would change the items behavior across your entire
applcation (and maybe even all of Office).  Instead create a
new command bar item and use your own code to do what you
want as well as what the real item does.

You could assign a macro to it.

I am not sure what you mean by "fire an event"

You can certainly have a custom button call VBA code (that normally how I
use custom menu items).

That code can most certainly "call" some event code you have in a form.

So, if you wanted to call the after update event of a particular control,
say "company" name after update event, the you could simply put a pubic
function in your form call

Public Function MyCoAfterCall

  Call CompanyName_AfterUpdate

end if

and, then for the custom menu/tool bar on-action, you go

=MyCoAfterCall()