If anyone know of a shopify app that does this, please advise! I have been looking into documentations and apps for this particular feature, but no luck. If any questions, please let me know.
You could set this up with Shopify Plus. Shopify Plus has a feature they call "multi-pass" that allows external login. Part of the handshake there is an email address but if your external system doesn't have that you can fake one.