You could manually setup a FTP account on the cPanel DNS-Only server and then choose the remote FTP backup option in "WHM >> Backup Configuration" on the cPanel server.
Note that FTP functionality is not integrated into cPanel DNS-Only by default. However, you are welcome to install a system package such as PureFTP through YUM and configure it manually. Searching for "YUM Pure-ftpd" on Google yields several tutorials on this that should be of help to you. Ensure you review /etc/yum.conf and make sure Pure-FTPd isn't on the "exclude=" line.