Can I use FTP accounts to authenticate over SFTP?

No, this is not possible. SFTP is an SSH-based protocol, and FTP is entirely separate.

To use SFTP, you will need SSH/Shell Access on your cPanel account, and you will only be able to authenticate with the cPanel account credentials or the server's root account credentials.

More information:

• How to Configure Your SFTP Client
• Manage Shell Access
• FTP Accounts