mail chimp is an email blaster so I don't think many people will want to help you subscribe people to spam.. but here is another suggestion.
Setup your own mailing list on your site. At least then you have half a chance of people helping you to do what you want because the database will be on the same server. Anything to do with mail chimp will require someone here to sign up, write code using cURL, test, retest, retest again etc. I really don't see that happening for free.
If you're looking to do it yourself, you're going to need to setup a cURL handler for a http request and then login to mail chimp, browse to your contacts page, submit the email by post and then logout - all while handling the cookies (so you're going to need a pretty solid understanding of those plus request and reply headers too).
My helpful sig has gone because a mod below the administrator gave me an infraction - despite the administrator personally agreeing to it.
If you need any php tips or tricks you can PM me.