Adding external wired NAS to Time Capsule First, I have none of this equipment, so I can't just test it and see ..Yet.
I Know I could hack a Time Capsule into a RaidBox via SATA/eSATA connector, But that's not the route I'd like to go.
If you plug in a NAS to a A1470 TC does acknowledge it's existence ? And Can you store to it?
I want to get a TC A1470. replace the small drive with a larger TB unit. Use that for my Time Machine Backups. Then add a 16TB NAS (wired) to the ethernet on the TC. Like a Drobo 5-Bay DAS Storage.
Mac, Mojave, External NAS, 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.