Here you can find a guide on how to create a cronjob to use sa-learn to teach spamassassin. The step #1 guides how to create imap folders for it. To do a bulk update on your server with a...
How to make Exim to send messages with IP assigned to other users is described here. And here in the article you can find some bash scripts for generating files with IP data automatically.
File uploading in a Directadmin plugin is possible, and there is nothing difficult if to say in general. Let me show some basic moments with the help of which you can achieve the desired.
Since directadmin version 1.453 there is a great feature available for plugins coders. Since now (01.07.2014) plugins have the ability to run as a specified User.