First of all add the site map to an unmanaged solution and export the solution, open the customizations.xml file in Visual Studio, associate it with the customizationSolution.xsd located in the SDK\Schemas folder in the SDK download package. To associate the customizations.xml file with the xsd file: in Visual studio press F4, this will show the property pane, click on schemas then "...", click add..., navigate to the folder select the file and click OK.
In order to be able to show a Silverlight application in the main page grid we need to create a web resources for both the Silverlight xap application and the HTML web page that hosts the Silverlight app (chek this post on how to create a Silverlight web resource) then reference the web page from the site map with the "$webresource:" directive see example below.
These few lines of code add a new group within the Workplace area and a new sub area within the group.
And this is the result, hope this helps someone.