You are not thinking about your issue with the navigation bar correctly. You do not need to add certain sub-items depending upon the current page. You need to hide certain sub-items. Plain HTML/CSS example:

   <style type="text/css">
       ul li ul {display:none;}
       #active ul {display:block;}
   </style>

   <ul>
      <li id="active">Section 1
         <ul>
            <li>Subsection 1.1</li>
            <li>Subsection 1.2</li>
         </ul>
      </li>
      <li>Section 2
         <ul>
            <li>Subsection 2.1</li>
            <li>Subsection 2.2</li>
         </ul>
      </li>
   </ul>
[download]

Renders as:

• Section 1
  • Subsection 1.1
  • Subsection 1.2
• Section 2

Even though this contains both submenus, only the first one gets displayed. Without knowing exactly what the generated HTML of you navbar looks like, I can only point to the general technique ...