Those are just convenience controls. To make custom links you can use plain html
<a href="/contact.aspx">Contact Us</a>
where / represents the root of the site.
If you want to add it into the links at the top you'll need to wrap it in an li element and add the css class sitelink like this:
<portal:WelcomeMessage ID="WelcomeMessage" runat="server" RenderAsListItem="true" ListItemCss="firstnav" />
<li><a class="sitelink" href="/contact.aspx">Contact Us</a></li>
Hope that helps,
Joe