Create new toolbar buttons

You can either alter the existing plugin or create your own one. But the process is techncially the same.

take the existing code:
ckeditor/_source/plugins/horizontalrule/plugin.js

copy the plugin and name it something snazzy like

ckeditor/_source/plugins/horizontalrule2/plugin.js
change the var pluginName = 'horizontalrule2';
and var horizontalruleCmd to var horizontalrule2Cmd (just so you know what you are doing and change it's functionality

upload it to /ckeditor/plugins/horizontalrule2/plugin.js
and then you need to tweak the config options to load it up
I do this in by CKEDITOR.Replace

and you need to add HorizontalRule2 to your toolbar.

<script>
CKEDITOR.config.toolbar_MA=[ ['Source','-','Cut','Copy','Paste','-','Undo','Redo','RemoveFormat','-','Link','Unlink','Anchor','-','Image','Table','HorizontalRule','HorizontalRule2','SpecialChar'], '/', ['Format','Templates','Bold','Italic','Underline','-','Superscript','-',['JustifyLeft','JustifyCenter','JustifyRight','JustifyBlock'],'-','NumberedList','BulletedList','-','Outdent','Indent'] ];

CKEDITOR.replace( '".$id."',
            {   toolbar:'MA',
                extraPlugins: 'horizontalrule2'
            });

</script>

as below:

(function()
{
   var horizontalrule2Cmd =
   {
      exec : function( editor )
      {
         editor.insertElement( editor.document.createElement( 'hr' ) );
      }
   };

   var pluginName = 'horizontalrule2';

   // Register a plugin named "horizontalrule2".
   CKEDITOR.plugins.add( pluginName,
   {
      init : function( editor )
      {
         editor.addCommand( pluginName, horizontalrule2Cmd );
         editor.ui.addButton( 'HorizontalRule2',
            {
               label : editor.lang.horizontalrule,
               command : pluginName
            });
      }
   });
})();