Setting up CrashPlan with NAS drive attached to Airport Extreme?
You should be able to use the drive on the Airport Extreme as a destination folder for your CrashPlan backups. CrashPlan can backup to a local folder, or to an external hard disk in addition to their online service. You should be able to treat the NAS in the same way as you treat an external hard drive, from CrashPlan's point of view. At least, that's my guess, but I haven't tested this.

This assumes you want to backup from your computer TO the NAS, and not that you want to backup FROM the NAS to CrashPlan's servers (although I'd imagine both are possible).