Custom CMS Database: Migration into Drupal Database

We are looking for a script that migrates data from a MySQL database created for a news site built with Code Igniter into a new installation of Drupal (version: 7.8). The Code Igniter database contains several tables, but only the tables pertaining to the articles (News, Categories, and Authors) are to be migrated. We need the ability to migrate content based on the date of creation of the article (only for the News table), so that we can choose a date range of articles and migrate only those.