There are very limited resources online describing how to migrate from Serendipity to Drupal. Among them, most popular articles are these two blogs:
- http://vhata.net/blog/2007/03/27/converting-from-serendipity-to-drupal
- http://blog.liip.ch/archive/2009/06/18/import-serendipity-entries-into-d...
However, as you can guess from the URLs, there blogs are too old and are probably for old versions of Drupal like Drupal 5 or Drupal 6.
In January 2013, Drupal user deminy wrote a script to import Serendipity blogs, comments, categories and tags to Drupal 7. For details, please check https://github.com/deminy/s9y-to-drupal.