Append a single icon to the toolbar

Did you implement this plugin or you're just using it?

If you implemented it, then of course you have full control over where the button will be added. You control this by the toolbar property of the button definition. I cannot find it documented, but basic usage is show in the plugin tutorial, so you specify just the toolbar group name in which this plugin will be placed. You can read more about groups here: http://docs.ckeditor.com/#!/api/CKEDITOR.config-cfg-toolbarGroups. However, as you can find e.g. in addButton documentation, you can also specify the order which will be taken into consideration when sorting buttons in a group. You can also create new groups.

If you're just using someone's plugin and you want to move one button, then you can of course reconfigure entire toolbar using either config.toolbar or config.toolbarGroups. These are the standard ways. However, you can also override button's toolbar property and if you do that in correct moment, the button will be assigned to different group:

CKEDITOR.replace( 'editor1', {
	on: {
		pluginsLoaded: function() {
			this.ui.items.Bold.toolbar = 'insert,100';
		}
	}
} );

Now, the bold button will assigned to "insert" group and will be placed at the end of it.