DISCLAIMER:
This has been only tested with a ReadyNAS Duo Ultra 2 and ADrive.com as cloud storage provider.

Enable root access (this may void your support from Netgear):
http://www.readynas.com/?page_id=608

SSH into your ReadyNAS and run:
cat .ssh/id_rsa.pub

Import the output into your cloud storage provider accepted public keys.

Append port in address field:
rsync.adrive.com:22

Use any username/password required by your cloud storage provider.

IMPORTANT: Don't click "Test", this makes the GUI hang in my case.

Set the schedule and options that fit your needs.

Click "Apply".

Your backup job should now be added successfully and you can start it from the "Backup jobs" menu.

Happy syncing! :)