TRAP:

As each customer is assigned a customer ID which is an auto incremented value you will need to take care in doing this so that your ID's remain unique and not the value you currently have in your csv which may already belong to another customer

It might be better to to install the news letter subscribe 3rd party module that only records as I recall the email address
You could then email all these individuals as a reminder that their email address is in the DB and if they were to create an account using that email they only need to enter a password plus the other details

Or if you have many, a better option is to start using one of the mass email services....one is mail chimp that is now allowing 2,000 addresses at no cost to you
You then have access to professional looking email templates and you will also appear more professional