It is possible to include a .t3d export file in an extension. This will be imported when the extension is installed. Such an extension is then called a distribution (see the Introduction Package in the Extension Manager).
The big drawback is that the .t3d export will only be imported at the first installation of the distribution. If you update it and install the updated extension the export file will not be imported anymore. This is done to prevent the page structure and content from appearing multiple times in an installation.
It would be a nice idea for a site kick starter, but since one of the goals of this sitepackage is to make it easier to update an existing site in a single operation this wouldn't really work.