I am able to add custom links to Magento's top.links with the following code that I save in ../myCustomTheme/layout/local.xml
<reference name="root">
<reference name="top.links">
<action method="addLink" translate="label title">
<label>example</label>
<url>example</url>
<title>example</title>
<prepare>true</prepare>
<urlParams helper="core/url/getHomeUrl"/>
<position>100</position>
<liParams/>
<aParams>class="top-link-example"</aParams>
<beforeText></beforeText>
<afterText></afterText>
</action>
</reference>
</reference>
The above code will create a link named example that points to http://myexampledomain.com/example. If I change this line of code
<url>example</url>
to
<url>http://myotherexampledomain.com</url>
I end up with a link named example that points to http://myexampledomain.com/http:/myotherexampledomain.com. I have tried setting the prepare parameter to false and adding various parameters to urlParams by looking at ../app/code/core/Mage/Core/Model/Url.php to no avail.