How to import customers from an external system, which stores the passwords hashes without a salt?

You need:

1. Add an attribute to the customer entity type, which will contain the information whether a customer is imported from an external system (which stores the customers password md5 hashes without a salt).

• Import the customers from the external system.
• Create a plugin to the standard customer authentication procedure, which will check whether the currently authenticated customer is impoorted from the external system, and will authenticate it without a salt.