MG Studio doesn't fully support the use of the new Composite Symbols from XML. The import should have been creating regular symbol library instances instead. I don't believe it's possible to convert the XML symbols to the standard Symbol Library. I believe you can work around this by opening one of the published layers from Map that uses one of the new Symbols, copying the XML, open your other layer and save as XML, edit the other layer replacing the point geometry symbol section and uploading the layer via the Web Tier test pages: http://servername/mapguide2009/mapagent/index.html using the SetResource command.
Regards,
Dave