You would need to research the encryption algorithm that Microsoft used in SQL 2000 and build your own decryption process and have it decrypt the values then reencrypt the values using a normal supported encryption method. Given that SQL 2000 is 14 years old I'm sure that this information is out there somewhere.

Another option would to force all your users to reset their passwords. I would highly recommend changing the encryption that you use to one of the supported encryption options as you do this.

Your only other option is to continue to run the application in SQL Server 2000.