In France and Spain all electrical wiring is done by using flexible plastic tubing (called Gaine in France) through which the wires are pulled with a 'tire-fils'. It is very simple to add extra cables when you want.
Have a look
here at the type of stuff. Really excellent.
It's the same here, except it's called preflex, and it ain't always so easy to pull another wire in. I should know, I work with the bloody stuff every day