howto add elements to sections of xml document

I apologize if this is a double-post, I did not
receive the message to my other address so I created
another one.

I would like to tranform the following xml:
<doc>
   <section>
      <title>XXXXXX</title>
      <para>XXX XXX XXX</para>

      <title>YYYYYY</title>
      <para>YYY YYY YYY</para>
      <para>YYY YYY YYY</para>
   </section>
</doc>

to:

<doc>
   <section>

      <sub-section>
         <title>XXXXXX</title>
         <para>XXX XXX XXX</para>
      </sub-section>

      <sub-section>
         <title>YYYYYY</title>
         <para>YYY YYY YYY</para>
         <para>YYY YYY YYY</para>
       <sub-section>

   </section>
</doc>


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