How can I sync additional user data with MailChimp?

The MailChimp Sync plugin is great, but I want to send more information about the user to MailChimp besides just their name and e-mail.

I would also like to have their username and site address.

I would like to be able to have an autoresponse go to the user from MailChimp after they create their site that has a welcome message including their username and link to their site.

I know, there is a welcome message within WordPress on the Network Settings page. However, I've found some users don't get that message and I'd prefer to have it go out from MailChimp since they will have better deliverability than I could hope to achieve.

I know there isn't currently a way within the menus to sync additional data, but is there a way I could edit the plugin code and add this functionality? I could poke around and guess but I don't want to break it so I was hoping to get some help.