Can I hack and extend my Macbook's power cord?

As you know if you have Mac, they come with a squared white charger that has a small adapter and also an adapter with a bulky power cord. As in: http://www.pollsb.com/photos/o/6619-macbook_charger.jpg

My question is, could I open, extend and rewire the thin cable (the DC part of the circuit), or is there any technical reason why it can’t be longer? http://aryanugraha.files.wordpress.com/2008/11/img_2047.jpg

Thanks!