An "encrypted folder" is really a file, mounted using the loop device. So it has a file system when mounted and you can share the mount point via NFS. The files inside are not encrypted. Do you need help creating this file and preparing the file system? Some distos have a graphic tool to do this for you. If your backup server has a GUI partitioning tool, look there. It will create and prepare the file, as well as write the /etc/fstab entries for it. All that remains is to share the mount point.
To do this manually, you would ue the "dd" command to create a large file; "losetup" to attach the file to a device; cryptsetup to prepare the encryption; and mkfs to create a file system on encryption device. More steps to create it initially, then to use it. Using your system's partitioning tool would be easier.
Here is a how-to on passwordless encryption: http://wejn.org/how-to-make-password...ryptsetup.html
The key is located on a USB stick. To locate the key on your backup server would be pointless, defeating the purpose of encryption.