@adrian2k7
Yes, you can do this.
You can use PHP code on the same site to do this: http://tribulant.com/docs/wordpress-mailing-list-plugin/3433
Or alternatively you can use the API to create an HTTP or CURL connection to the site where the WordPress Newsletter plugin is installed. You can use the subscriber_add API method and the API key and details is under Newsletters > Configuration > API inside the plugin. Here is documentation on that: http://tribulant.com/docs/wordpress-mailing-list-plugin/8538
Let me know if this helps.
Thank you. Works fine for me.
Exactly what I waas looking for (forst solution).