Using Time Machine with an Airport Extreme AirDisk
Q4. How do I back-up the Air Disk connected to my Airport?
If you have other data, besides Time Machine backups, on disk(s) attached to your Airport, Time Machine cannot back that data up.
In a pinch, you can eject and disconnect a USB drive from your Airport, attach it to your Mac, and copy or back it up that way.
If you want to back up your Time Machine backups, or have Time Machine back up to two separate destinations, that’s difficult and not recommended. See Time Machine FAQ #27.
Other options are the CarbonCopyCloner (about $40 with a 30-day free trial) or Chronosync (about $40) apps.
Time Machine can back up to a USB drive 
connected to the new, tall Airport Extreme "ac" models (6th generation) introduced in June, 2013.
But it's not reliable and not supported by Apple on the older, flat models. They have different hardware. If there's trouble with it, you won't get much, if any, help from Apple. If you decide to take the risk, you'd be well advised to have other backups; don't depend on these 100%. See Time Machine FAQ #2 for supported Time Machine destinations.