I ran into this issue early on as well.
Unless something has changed, this was what was concluded for me:
There is no support for 3rd-party authentication or user migration.
However, you can use the order webhooks to catch orders and store them in your own database keyed to your users. Personally, I use Auth0 for authentication and I use Firebase to store orders belonging to my users.
The only key limitation of this that I’ve run into is that you can’t store the customers’ payment details because their account isn’t through Snipcart.
As an aside, if anybody from Snipcart reads this… I attempted to provide links to the relevant snipcart documentation but received an error messaging indicating that links to the “docs.snipcart” domain are not allowed.