There used to be someone on this Board who did it at Chicago, but I think its a huge waste of money. I can't see any good reason to add another year to law school regardless of how much of a rankings jump you make.
You may want to consider visiting at another school for your third year. This will typically give you access to the school's OCI and career services office, although you will still graduate from your current school.
Good luck with whatever you decide, but I think adding another year to your JD is a big mistake.