Combine Newsletter plugin with subscribe form

Background:
I am using the WordPress Newsletter plugin to send out monthly enewsletters.

I am also using a simple form where users can subscribe to the print magazine and make a payment using their credit card. At the bottom of this form is a checkbox that asks the question “do you wish to receive our monthly enewsletter?”

Users can either subscribe to the enewsletter from a small text box asking for their email AND/OR fill out a seperate form if they wish to subscribe for 12 months to the print magazine.

Problem:
How do I update the Newsletter subscribe list from an external form. Basically I want to be able to automatically update the enewsletter subscriber list within the Newsletter management admin area whenever someone clicks “yes” on the checkbox asking to recieve the enewsletter from the seperate form.