They would need to make a licensing agreement with Image. You might have some luck pushing for a company to do something already within the sphere of its licensing agreements (particularly Bowen), but I really doubt you can convince a company to pick up a new license to make one statue.
As for Sideshow generally, they do their own thing.
For one statue, probably not. . . but a series, with Lying Cat being the first? Marko and Alana, The Will (as a companion piece to Lying Cat), Prince Robot IV. How cool would a statue of The Stalk be? Done right, that would be awesome. An Izabel statue could have some incredible detail.
Who has The Walking Dead statue rights? I've seen some stuff come out (zombie head fish tank, a few Daryl, etc.). Is it Gentle Giant? They would have an "in" with Image.