I think it's clear from the photo that the Eisenman-designed section of the building, at least, was conceived as a sculptural whole. In other words, if you pull the front off and add a couple stories without also altering the "main convention space", the logic (and architectural value) of the whole thing is irrevocably destroyed.
Personally, I hate the appearance of every Eisenman building I've seen in person*, but when they added the non-Eisenman-designed section on the north end of the convention center, I had to admit that it looked completely pedestrian compared to the original part of the building, even as it tried to use the same visual vocabulary. I can only assume that they tried to make Eisenman's original "impracticalities" more practical. The convention center as it is might not be to my taste, but at least it doesn't look like a mall. I don't think Columbus really needs to have its few pieces of distinctive architecture messed with.
*I also hate the way the insides function, but that's not germane here.