Are you sure its not PAS equipped? i thought all Clio II's has assisted steering but I've been wrong before when it comes to spec and equipment.
It can be done yes but it really depends on the engine in the car as its not as simple as you may think. Firstly, you will have to remove your manual setup:
Steering rack
Steering column
(I'd be tempted to stick it on the CS forsale section to as the track boys/girls would love a proper manual setup)
Once that's all gone you need a new steering rack and steering column for a hydraulic/ePAS setup (dealers choice) and then add the mountings for any pumps along with the pump, pipes and fluid reservoirs to the engine bay (or the circuits/fuses/wiring if you choose the ePas route).
Personally I'd say it would be cheaper to buy her a car with PAS already than retro fit as the labour will be considerable. Or, even if you're doing the work yourself the parts are likely to come to somewhere in the £200-280ish region.
Mick