Hi Petter
Yes it is quite possible to convert to PAS, you will need to change the sterring box, the idler, steering track rod ends, possibly the sterring side rods, and I am not sure if the crankshaft pulley will need changing or not.
You will also need the PAS reservoir, pipes, pump and fixing brackets (all mounting points should be on the car).
I would imagine that the work will require patience as there is a lot to do and not much room to work in!
Is your car LHD or RHD? Probably easier to get a scrapcar & remove all the parts you need that way you can be sure you get all the parts you need, I believe UK suppliers found on the links at this web site can provide the bitsyou need.
Good luck
Gary