How can I send data to an additional field in mailchimp?

I'm not very savy with php, or other web languages, but have been able to learn a little as I go.

I just installed and successfully tested the mailchimp plugin and a new user I create in WordPress gets added to my mailchimp list.

How can I also fill in another field in mailchimp that is not a field that in wordpress? For example, I added this line in the signup box on a web page to track where a person added their name to the list (changing "pagename" to reflect the page the form is on):

<input type="hidden" name="SIGNUP" id="SIGNUP" value="pagename" />

I would like to have all names added to mailchimp by wordpress by the plugin to have a value of WP in the SIGNUP field.