MailPoet Paid Memberships Pro Add-on: import custom fields?

I have created some custom fields in PM Pro and would like to get that data imported to the equivalent custom fields in Mailpoet. I’m using the very cool “MailPoet Paid Memberships Pro Add-on” plugin, and even got into the code, but have not been able to figure out how to import custom field values. Any thoughts on how best to proceed?

https://www.ads-software.com/plugins/mailpoet-paid-memberships-pro-add-on/