Disclaimer: i don't have a firestick or an xbox one.
So Kodi is running on your firestick which you have plugged into the xbox one
hdmi input port.
Your xbox one must then be connected to the TV via the other
hdmi output port.
That way you will see the firestick video output
In this sence, the xbox one is simply acting as a hdmi switch of sorts (though xbox one may also do some scaling of the firestick video output but i'm not sure of this).
As such, the kodi video output is taken care of...
But the question you should ask yourself is how can the firestick access files on a hdd connected to another computer.
In your specific case, the other computer is in fact your 'locked down' xbox one.
Obviously some sort of network is required to share the files on a usb connected hdd on another computer.
Such a networks could be constructed via wired ethernet or wireless 80211 or a combination of both
I'd guess, as your firestick could be connected to your network, and as such it should be able to use nfs to access the files on such a hdd.
However, the xbox one may or may not make it's external usb hdd accessable to other computers on your network via nfs or other methods.
MS may simply like things locked down...
As such, you need to look through the xbox one menus and hunt for some network sharing setting associated with the external hdd.
Hopefully you will see something and it will provide some data associated with sharing this resource when selected.
Then you need to specify this sharing data within Kodi itself that is running on the firestick.
As mentioned, i don't have either a firestick or xbox one, i can't be specific... but Google may help find a solution