The default directory structure for new Projects is specified by a set of values in the registry.
Look under the key HKEY_LOCAL_MACHINE\SOFTWARE\Softimage\SOFTIMAGE Application\C:|Softimage|Softimage_7.5|Application|bin\Default Directory Structure.
You can add new directories by adding a new string value to the Default Directory Structure registry key.
A plug-in that updates the default directory structure was posted to the XSI Mailing list .
Also posted in the Autodesk KB.