Hi,
I can write a query to export your Pligg data and then write another query to import it to your existing autoresponder, presuming that you have PHPMyAdmin access to both your Pligg database and your autoresponder database.
I'm unsure what you mean about adding the autoresponder code to your content management system.. normally users sign up via the front end, rather than the back-end?
What autoresponder software/service are you using if it's not hosted by you?
Thanks,
Anthony