Add div after each field of a certain name
Copy modules/field/theme/field.tpl.php file into sites/all/themes/YOUR_THEME/templates

Rename it to field--additional_content--YOURCONTENTTYPENAME.tpl.php
(or only field--additional_content.tpl.php if you want to change its template for all content types)

Edit its template like this:

&lt;div class="&lt;?php print $classes; ?&gt;"&lt;?php print $attributes; ?&gt;&gt;
  &lt;?php if (!$label_hidden): ?&gt;
    &lt;div class="field-label"&lt;?php print $title_attributes; ?&gt;&gt;&lt;?php print $label ?&gt;:&amp;nbsp;&lt;/div&gt;
  &lt;?php endif; ?&gt;
  &lt;div class="field-items"&lt;?php print $content_attributes; ?&gt;&gt;
    &lt;?php foreach ($items as $delta =&gt; $item): ?&gt;
      &lt;div class="field-item &lt;?php print $delta % 2 ? 'odd' : 'even'; ?&gt;"&lt;?php print $item_attributes[$delta]; ?&gt;&gt;&lt;?php print render($item); ?&gt;&lt;/div&gt;
      &lt;div class="your-additional-div"&gt;&lt;/div&gt;
    &lt;?php endforeach; ?&gt;
  &lt;/div&gt;
&lt;/div&gt;