Get the indexen.html and indexfr.html files in the home or acceuil folder for each site
find the unique string that identifies the last list item in the footer
replace it with itself + an additional list item for the new link
Do the same search and replaces, but this time adding an additional "../" before the pathname in the a link of the existing last list item.

Some sites use actual LI, some use plain text to render the footer list
Some sites UPPER CASE, some Title Case for the text values in the list

Example site using plain text:

search for
| <a href="../archives/indexen.html">ARCHIVES</a>
replace with
| <a href="../archives/indexen.html">ARCHIVES</a> | <a href="http://canadianmysteries.ca/en/becomingHistorian.php">BECOMING A HISTORIAN</a>
search for
| <a href="../../archives/indexen.html">ARCHIVES</a>
replace with
| <a href="../../archives/indexen.html">ARCHIVES</a> | <a href="http://canadianmysteries.ca/en/becomingHistorian.php">BECOMING A HISTORIAN</a>
search for
| <a href="../archives/indexfr.html">ARCHIVES</a>
replace with
| <a href="../archives/indexfr.html">ARCHIVES</a> | <a href="http://canadianmysteries.ca/fr/devenirHistorien.php">DEVENIR HISTORIEN</a>
search for
| <a href="../../archives/indexfr.html">ARCHIVES</a>
replace with
| <a href="../../archives/indexfr.html">ARCHIVES</a> | <a href="http://canadianmysteries.ca/fr/devenirHistorien.php">DEVENIR HISTORIEN</a>

Example site using li.last-footer

search for
<li class="last-footer"><a href="../archives/indexen.html">Archives</a></li>
replace with
<li><a href="../archives/indexen.html">Archives</a></li>
<li class="last-footer"><a href="http://canadianmysteries.ca/en/becomingHistorian.php">Becoming a Historian</a></li>
search for
<li class="last-footer"><a href="../../archives/indexen.html">Archives</a></li>
replace with
<li><a href="../../archives/indexen.html">Archives</a></li>
<li class="last-footer"><a href="http://canadianmysteries.ca/en/becomingHistorian.php">Becoming a Historian</a></li>
search for
<li class="last-footer"><a href="../archives/indexfr.html">Archives</a></li>
replace with
<li><a href="../archives/indexfr.html">Archives</a></li>
<li class="last-footer"><a href="http://canadianmysteries.ca/fr/devenirHistorien.php">Devenir Historien</a></li><li class="last-footer"><a href="../../archives/indexfr.html">Archives</a></li>
search for
<li class="last-footer"><a href="../../archives/indexfr.html">Archives</a></li>
replace with
<li><a href="../../archives/indexfr.html">Archives</a></li>
<li class="last-footer"><a href="http://canadianmysteries.ca/fr/devenirHistorien.php">Devenir Historien</a></li>

Example site using li

search for
<li><a href="../archives/indexen.html">Archives</a></li>
</ul>
</div>
</body>
replace with
<li><a href="../archives/indexen.html">Archives</a></li>
<li><a href="http://canadianmysteries.ca/en/becomingHistorian.php">Becoming a Historian</a></li>
</ul>
</div>
</body>

search for
<li><a href="../../archives/indexen.html">Archives</a></li>
</ul>
</div>
</body>
replace with
<li><a href="../../archives/indexen.html">Archives</a></li>
<li><a href="http://canadianmysteries.ca/en/becomingHistorian.php">Becoming a Historian</a></li>
</ul>
</div>
</body>