I like MailChimp and Aweber. MailChimp is free up till 500 subscribers, after that the pricing is pretty much on par with Aweber.
Yahoo doesn't just allow to take over a group? That's too bad, because that would've been your easiest route.
Any CAN-SPAM compliant email marketing solution (all the good ones) is going to make those email addresses you've just obtained re-optin, so I'd expect you to lose quite a bit of the subscribers now.
Google Groups allows importing in of email addresses, but you run in to issues of those people did not optin to that group, and you could end up getting in to trouble for those kind of things.