Update custom field by API or hook

Hi there,
I’ve developed a special registration form on which users can also opt-in for different MailPoet lists. The procedure is that anybody can apply for an account, but it is pending. After manual review the user will get either approved or rejected. In case of an approval, the user will be subscribed to the selected MP lists using the API. As the user will always get automatically added by Mailpoet to the MP wordpress user list (meaning that he always exists when my afetr-approval function runs), I use the $mailpoet_api->subscribeToLists() function. That all works well. The only thing is that I haven’t found a way to update subscriber fields after the auto creation by MailPoet. I have custom fields set up on the regular MP form, which I would like to populate by my program. As there is no API function for updating subscriber data or a hook that could do that, has anybody an idea how to achieve that?

Thank you for your help.

regards, Rado