Let's assume an existing database of customers outside Shopify. Would it make sense to check upon registration in the Shopify store, whether the user already exists in the external database and maybe link these two accounts from the Shopify customer database and the existing external one? In which database would you ideally store the password for the Shopify account then? What is common practice in this type of cases? Is this even supported by Shopify (through API or custom APP)? I'm sorry for the multide of questions but I need a bit of guidance. Thank you very much