To make a US spec 4WS you will need a new front steerin rack with output for 4ws, front to rear steering linkage, rear steering rack, new rear suspension frame, electrics and hydraulics to make it work. Certainly not worth it. The rear rack alone would cost about $2000USD to buy new from mazda, let alone all the other hardware. You may be able to find all the parts 2nd hand, but as to wether it will work or not , who knows.
Better off buying it already installed on an MX.
Trev