Filling spare FX CF 480cc bottle

I am sure you can, you just have to get an adapter right sized what screws onto the bottle with the fill line.
the high pressure on outside will open the one way/checkvalve in the neck. Airforce guns used to fill like that off the gun.
probably it ill have to have outer threads.

the main problem what you may face, that if you take the bottle on off back on many times, there will be wear on the gun side too.