I was sent here by Shopify Plus Support which promised me I would find answers to my questions. here. 
So here we go:
We are migrating subscriptions from woocommerce with paypal reference transactions. From shopify we recevied two sample import files: one file has the customer address data and line items, one item per line. the other file has only two columns, one is an email adress and the other one a “paypal token”.
My issues/questions:
a) it is common that customers use one mailadress for their order and a different one for their paypal account. Therefore I would need to correlate a customer mail address to a customer paypal address. However, given those two files I do not see how I can correlate this?
b) Nobody seems to know that the “paypal token” is. Do I understand correclty that this is one oe the last transaction IDs of the paypal customer whose email i give?
I also found those two doc links, but they do not provide enough infos about this either. And I would need to make my own api importer, while I understand that Shopify offers a migration service with those import files. So I would prefer to migrate like this.
Thank you ! Matthias
Migrate customer information to Shopify
customerPaymentMethodPaypalBillingAgreementCreate (shopify.dev)