combine normal php site with codeigniter

[eluser]manik005[/eluser]
i already have a site that was built with normal php . But now i wanna add some more features like fb connect and so on to it. I wish to use codeigniter for these new features. In that case should i convert the old site into codeigniter and then add new feature or is there any other way to add just the new features with codeigniter?