Valid for Sitecore 
  5.3
Adding Tab to Content Editor Taskbar
  Q:  
  Is it possible to add a new tab to the Content Editor taskbar? 
  A: 
  Follow the steps below to add a new tab for the Security Editor application on the Content Editor taskbar (see the screenshot below): 
 
  
  
    - Switch to the Core database.
    
- Create a new item named 'SecurityEditorForm' under Sitecore » System » Settings » Content Editor » Applications.
   
- Point the Source Field value to the ASPX page which defines the tab (see below). 
Creating the .html file for the tab:
  
    - Use the following page: \sitecore\shell\Applications\Workbox\Default.html as a basis for your .html page. (You may create any other page of your own or modify the existing one)
    
- Locate the <td id="WorkboxFrame"> section on the newly created page and replace the existing line with the one emphasized in bold below:
    <td id="WorkboxFrame">
 <iframe src="default.html?xmlcontrol=SecurityEditor" width="100%"
 height="100%" frameborder="0"></iframe>
 </td>