Figured it out!
I edited the file “wp-super-edit/tinymce_plugins/advlink/link.htm”
I changed lines 197-219 from:
<td><label id="rellabel" for="rel">{#advlink_dlg.rel}</label></td>
<td><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>
</td>
to
<td><label id="rellabel" for="rel">{#advlink_dlg.rel}</label></td>
<td><input type="text" id="rel" name="type" value="" /></td>
I can now input my own link relationship!