If you head down to halfrauds or any other car parts shop, you can buy aftermarket kits to give you a socket, all you need to do is ensure you get it linked into a suitable fused power supply, either tap one off the fusebox, or run cables straight to the battery, this would give you constant power rather than have it going off with the ignition.
If you do run a cable to the battery, make sure it is suitable to carry the load your are likely to put on it (depending on what you are going to plug in (eg a fridge would have a bigger current drain), and add an inline fuse into your wiring.
This would be quite qimple, you would spend longer trying to work out where you are going to put the socket.