Wordpress is a popular Open Source application to run a website. There are several sides to making phpList and Wordpress work together:
This is some system that allows you to have a “sign up to our newsletters” widget on your Wordpress site, which then subscribes the person to your phpList installation.
Implementing this will require digging deeper into the Wordpress mechanism to handle “users”. It may mean integrating calls to phpList at common stages of the Wordpress system. For example when someone makes a comment on a post they could be added to a list in phpList. This could be always, or only when they check a checkbox.
Why create a template creation system in phpList if Wordpress already does that. phpList is very good at sending a webpage, and Wordpress designers know how to create templates in Wordpress. Why not combine the two and have designers create a newsletter theme in Wordpress, editors edit the newsletter in Wordpress and then at the end post the URL to phpList for sending.
There are several plugins available that work in Wordpress to sign up to phpList. The most recent addition can do this very neatly using the Ajax subscribe functionality of phpList.