On the PhPL33t blog, there's a guide for automating the creation of email addresses with the Plesk administration application:
In 2003 I wrote "Plesk Auto Email", the first fully funtional Plesk email automation suite. Now, I am going to show you how to do it. You will need a dedicated server with Linux, Plesk 7.5 and higher, php5, mysql5, and root access. This will show you how to code the auto creation, I am not going into deletions and edits at this time because I don't have all night to blog.
Included are the database table to store the commands in, the PHP script to create the commands (the integration into the other piece of software) and the PHP cron script that looks in the command table and executes each as it goes through.