You could add their custom code to the custom area of the plugin admin.
Something like this works for my site:
<!-- AddThis Button BEGIN -->
<div class="addthis_toolbox addthis_default_style ">
<a class="addthis_button_facebook_like" fb:like:layout="button_count"></a>
<a class="addthis_button_tweet"></a>
<a class="addthis_button_google_plusone" g:plusone:size="medium"></a>
<a class="addthis_button_stumbleupon_badge"></a>
<a class="addthis_button_pinterest_pinit"></a>
<a class="addthis_counter addthis_pill_style"></a>
</div>
@myrko: Sorry to miss this query. Thanks Martin for the reply. That’s exactly how you should add custom service.
