I made this customization in few seconds

Here is how to:
Find this location: tinymce_3_2_3_1\jscripts\tiny_mce\plugins\advlink\
And there is a file - link.htm
Open this link.htm in some text editors, for example notepad and find following code:
<select id="rel" name="rel">
<option value="">{#not_set}</option>
<option value="lightbox">Lightbox</option>
<option value="alternate">Alternate</option>
<option value="designates">Designates</option>
<option value="stylesheet">Stylesheet</option>
<option value="start">Start</option>
<option value="next">Next</option>
<option value="prev">Prev</option>
<option value="contents">Contents</option>
<option value="index">Index</option>
<option value="glossary">Glossary</option>
<option value="copyright">Copyright</option>
<option value="chapter">Chapter</option>
<option value="subsection">Subsection</option>
<option value="appendix">Appendix</option>
<option value="help">Help</option>
<option value="bookmark">Bookmark</option>
<option value="nofollow">No Follow</option>
<option value="tag">Tag</option>
</select>
And insert this line after <select tag........
<option value="prettyPhoto[pp_gal]">prettyPhoto</option>
Hope this will help, for me it worked well!
Enjoy coding
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