Yes, you can.

You would need to create and FTP account within you cPanel interface under 'Files' section. When you create additional FTP accounts, you can limit what directories they can access. When FTP accounts are created, each account is assigned to a directory or folder. When an account is assigned to a directory, it can reach that directory but cannot go above the directory it is assigned to.

When cPanel automatically created the FTP account for your Addon domain, it is assigned to that Addon domain;s document root directory.