Can *all* files on an AirPort Extreme be encrypted with FileVault? I have a Macbook Pro (Mac OS 10.10.2, late 2013 model) and am considering to buy an AirPort Extreme 2TB. I want to use it not only for TimeCapsule backups, but also as a general-purpose storage disk (as discussed here Can I use a Time Capsule as an all-purpose storage disk?).

Can the data, which I save on AirPort Extreme not in the context of TimeCapsule, be encrypted with FileVault?

My question is not if TimeCapsule-backupped data can be encrypted - as I understand this is possible - but my question is if the other data, put onto AirPort Extreme outside of the TimeCapsule functionality, can be encrypted with FileVault?

I.e. can the entire AirPort Extreme be encrypted with FileVault?
Yes, just create an encrypted disk image on the Airport Extreme. That has nothing do do with it being an Airport, or having Time Capsule functionality; you can do it on any disk that you can mount. It's not technically FileVault, but it is 128- or 256-bit AES encryption.