NationBuilder does not currently integrate with other CMS (content management system) platforms like WordPress or Drupal. NationBuilder is designed to actually replace those tools for a lot of people, and be a huge upgrade.
Big usability problems frequently arise in the interaction between activism tools, CMS tools, and social networking sites like Facebook, so creating a platform that was fully integrated was our first mission. This is part of what creates such a great user experience for you and your users.
But soon you will be able to take just the elements of NationBuilder you like and mix and match them with whatever other tools you may want to use.
If you want to use our action pages in concert with an existing site, here's some info on using redirects and action chains to integrate NationBuilder and another CMS.
And if you want to migrate an existing site to NationBuilder's CMS, check out our legacy URL importer.