Yes, I haven't tried it yet, but my guess is that you add this code
Facebook login code:
Code:
<xen:if is="{$xenOptions.facebookAppId}">
<dl class="ctrlUnit fbUnit">
<dt></dt>
<dd><a href="{xen:link register/facebook, '', 'reg=1'}" class="fbLogin"><span>{xen:phrase login_with_facebook}</span></a></dd>
</dl>
</xen:if>
to the sidebar_visitor_panel template, find:
Code:
<div class="section loginButton">
<div class="secondaryContent">
<label for="LoginControl" id="SignupButton"><span>{xen:phrase sign_up_now}</span></label>
</div>
</div>
and add the Facebook code below or above it.
hope that helps.