It's actually easy to do jelf, any database management software will do it including phpMyAdmin. The only catch is the passwords, which will probably end up useless.
If that happens, and if this is an active/current customer list, I'd recommend e-mailing them ahead of time to let them know that on x date their password will be reset automatically by the system, and the next time they log in just to hit "Forgot My Password" to get a new one.
Next, before uploading the CSV just set all the passwords to the same random value. Then upload it and let the customers who actually come back handle the change.
Frank