No, you cannot reverse power supplies, they simply do not work like that, you will probably do a lot of damage if you try. There are lots of invertors for sale that will give you mains voltage from a cigar lighter socket, you only need one rated around 100W, so should not be too expensive, then you could just plug your laptop PSU into this. I may be that you can buy a car adaptor for your laptop that effectivley does the same thing, but rather more efficiently. You are probably better off not trying to build one, if you are not certain what you are doing. Good luck