Can ordinary tap water be used to top up cooling system of Citroen Xsara Picasso ?

Other answer:

That depends on how much you need to top up. The mixture should be 50/50 antifreeze and water. It doesn't have to be exactly 50/50 but you should not deviate too much from that mixture.

Tap water isn't great as it has minerals in it. Distilled water is preferred.

You're not supposed to use tap water, because the minerals can cause rust. Buy a quart of distilled water at any supermarket. Ideally you would use 50/50 mix anyway.
If you mix it 50/50 with antifreeze you can use it. Don't just use straight water. it will cause the inside of the motor to rust up and plug up the radiator and heater core and it will also fry the water pump seal because the antifreeze has a rust inhibitor in it as well as a lubricant to keep the water pump seal lubed.
Yes in an emergency. Best to mix it 50/50 with antifreeze.
It would work in a pinch – but not recommended due to the mineral content – you would want to flush and fill in the near future, unless you are selling the car – then it would be someones else's problem and not likely to be detected.
Yes it can.