Pass custom fields as merge fields
Note: this functionality requires the purchase of the Custom Fields add-on too.
You can send custom fields values to your MailChimp lists.
Navigate to Users -> MailChimp and scroll down the page, you'll notice a "Mailchimp custom fields" table where you can configure your custom fields and merge fields.
Once merge fields have been loaded, press the "add entry" button and configure the custom field and merge field that you want to match. Upon registration and account update, WPUM will send the fields to your MailChimp account.