Adding external wired NAS to Time Capsule
If the NAS is connected over Ethernet then you are dependant on the NAS software being able to present a share that supports TimeMachine.
For example, the Synology series of boxes let's you create a share on any volume (Raid or non-Raid) and has an option to allow TimeMachine backups to be made to this share.
I have not seen a NAS that would present a drive / share via USB.  A simple external drive can connect to the AP via usb and be used for TimeMachine this way though.
You have to be careful moving the backups from one drive to another - the format changes from a direct connected USB drive to those connected via AP or NAS and the actual mount point name will change - the NAS share name can be totally different from the AP name it is connected to.
Be aware you cannot encrypt the data drive on the AP so you have to set the TimeMachine backups to be encrypted for security of data.