In 1.2, manually edit the live sitemap.
<node href="http://solprovider.com" id="solprovider" visibleinnav="true">
window.location = "http://solprovider.com";
In 1.3, external links are just another Resource. The editor for the "link" Resource type works well. [You should check if the rest of Lenya 1.3 has been completed. The XML editor (for documents) and Structure editor (for hierarchical sitetrees) are not ready as of 20060910.]