There are a couple of plugs on the side of the metal case. I always see if I can get the upper one loose before I undo the lower one. To refill, I've got a small pump that I bought at a marine store, used for filling lower units on outboard motors and inboard/outboards. It fits into a standard quart bottle, and then you just pump it into the transmission. I personally find this a whole lot cleaner and easier than using one of the "syringe" type units.(It works great for filling differentials, too.) I put a little thread sealer, or even never-seize, in a pinch (NOT teflon tape) on the pipe plugs before I reinstall them.
To answer your question about using the OD in any other gear, if the OD is hooked up like the factory intended, no. There is a "4th gear switch" on the tranny that only allows power to the OD unit with 4th gear selected. The incremental steps between 2nd, 2ndOD, 3rd, 3rdOD, 4th and OD are so close together it makes very little practical difference. If your vehicle doesn't have the factory set-up, UNDER NO CIRCUMSTANCES engage reverse with the OD on. This is the fastest single way to turn your OD into something ready for the scrap heap. If you haven't already, procure a Haynes manual for your 220. You'll find some great stuff in there! Happy motoring.