The Bulk Load client will not be able to decrypt an encrypted file. There are other possible ways to secure the data including limited access, encrypted file systems and other external access control list (ACL) protections.
The best option to achieve this functionality is to encrypt each field of the CSV file and ensure that the resultant file still fit the CSV rules. Then write a BLTH that would take the encrypted data of the information from the CSV and decrypt it. Then place the data into each field properly before the task submission occurred.