In theory,everything is possible! A power steering rack is a very simple hydraulic device. A competent machine shop can build the hydraulic rack ,rotary valve and assorted hosing and use a pump from some other car. For the price of an immaculate R4, should I add...
In modern cars that are not equipped as standard with power steering, we use to find a same donor car that had this option, then use all these parts straight from one car to the other. Even then, some fabricating is often needed. But since no Renault 4, 6, or Mk1 5 was originally equipped with power steering, you can't go that way.
Now a question. For what reason will you ever need power steering on a 650 kg vehicle fitted with narrow tyres and already excellent steering system???