Well, if you really want something that looks 'hammer' like, one of the Chaos Dreadnought CC weapons is a
hammer. You could probably find and modify that bit, or create something similar from scratch. Alternately, I've seen a few conversions where people have modified a standard dreadnought fist to be 'holding' a massive thunderhammer (but personally I've always thought these look awful).
What
I think would be really cool is going with more of a pile driver style. A long, heavy beam of metal that can be forced forward. Something like this (excuse the awful picture):
