Besides 'First Name', 'Last Name’, and ‘Email’, other fields are possible to be directly exported to Mailchimp.
You could use our zapier integration to achieve what you need. It's super flexible.
This is the template zap you can use:
https://zapier.com/apps/mailchimp/integrations/waiverforever/22735/save-waiver-signed-customers-email-into-mailchimp