Have you thought of possibly building a short wall to enclose the knee braces and then do the columns on top of the short wall? Second, why weren't the columns installed when the job was being done instead of afterwards??? Also, how do you plan to install the columns around the existing posts? If they're the hollow type columns that I'm thinking of, then there's no need for posts (they support a whole lot more weight than most folks think),
Let me know the situation, and we'll see what we can figut out.