yes http://www.pac-audio.com/products/productsCatagory.asp?mmSearch=Steering%20Wheel%20Interface#SWI-JACK
The same way you take it off! But in a reverse order.
on the steering wheel
Into the power steering reservoir under the hood.
Yes
Yes, i own one and the steering wheel locks on me sometimes on accident.
Yes, it's possible.
You don't have an airbag
Yes>>assuming you use the wire-adapter connection plugs sold seperatly
No. None of the Z32 300zx's have them. (1990-1996) There is no option for it either. The only thing you could do is buy an aftermarket wheel that is smaller.
In the hatch or behind seats
Automatic Transmission fluid
The power steering fluid reservoir is up on the passenger side wheel well in the engine bay. Looks like a short fat silver coke can with hoses coming out of it. On a side note, the 300zx twin turbo uses ATF (automatic transmission fluid) not power steering fluid. usually a green casing with black top. the following link is NOT a 300zx, but the reservoir is about the same. http://www.sillbeer.com/images/engine/power-steering-reservoir.jpg