I think it is easy to make - just chop off the axe blade and glue on something hammer-like. I have not done so myself, but one of my first conversions was modifying the axe of an Allarus Shield Captain to instead be a labrys, and that was pretty easy. It's a forgiving thing to modify.
Maybe you can make a sufficiently elaborate hammerhead out of a pauldron? Otherwise I'd just glue on two more axe blades, one on each side of the original, slightly angled, and fill in the resulting two gaps.