In the path:
you can find navigation.xml, pages.xml and portal.xml where you can modify the default portal pages navigation as a template, adding your own pages.
This could be the most easy way to modify pages in the portal via XML.
A most powerful way it can be to create an extension of the portal adding new sites and new pages (for example, the mobile site is designed as an extension), you can find details under the following path:
An extensions is a concept of a package container with your own portal configuration plus skins plus your own portlets, so you can take a look into the mobile one for more details.
A good point to start investigating with extensions can be GateIn quickstarts in the following link:
Hope this helps.