360 Harness

You should search under racing, there are a few threads on this topic. In short, the answer to your question is "yes". There are factory mounting points/kits etc that can be added.

However, one of the main reasons for mandated 3 point belts in regular cars is that you can slip sideways in the event of a roll-over and crushed roof, something you can't do with harnesses. The common wisdom is to at least add a roll bar also.

 

Here's one with some pictures.

http://ferrarichat.com/forum/showthread.php?t=113040&highlight=roll+bar

Lots of good debate in that thread.

Obviously in the Porsche world lots of people fit the half cage with belts. I know of a few people that have done some sort of roll bar in a CS.

I personally bought a race car with a full cage and kept the CS. That however is a rather more expensive option