I have the same question. And not encouraged to see 0 responses….
You can pass custom user meta/etc to MailChimp by using the pmpro_mailchimp_listsubscribe_fields filter here:
https://github.com/strangerstudios/pmpro-mailchimp/blob/master/pmpro-mailchimp.php
I don’t have a good example, but you basically update the $fields array passed in to have new entries. The key should match the field name at MailChimp and the value is the value you want.
I don’t monitor the support forums for pmpro-mailchimp here all the time. You can get a faster response and more help via the paid PMPro support: http://www.paidmembershipspro.com/support/