Yes; while using Weapon Versatility in that manner it counts as a piercing weapon, and thus qualifies for Hamatula strike. I'm not sure how exactly you'd impale someone with a hammer, but it works.
"Type: Weapons are classified according to the type of damage they deal: (...)
In other cases, a weapon can deal either of two types of damage. In a situation where the damage type is significant, the wielder can choose which type of damage to deal with such a weapon." CRB pg. 144
In other words, the classification is done at the time of attack. If you attack with the hammer to deal piercing damage, it counts as a piercing weapon, includign for effects like Hamatula Strike.
Be prepared for disagreement. I am of the opinion that while YOU may be able to do piercing damage with a hammer due to weapon versatility, the weapon has not changed. It is still whatever sort of weapon it is when you are NOT wielding it.
Weapon Versatility states only that the damage is changed. Had Weapon Versatility had some different verbage in it like Bladed Brush that the weapon is treated as a piercing weapon, then you might have a case.
Be prepared for disagreement. I am of the opinion that while YOU may be able to do piercing damage with a hammer due to weapon versatility, the weapon has not changed. It is still whatever sort of weapon it is when you are NOT wielding it.
That's true, but irrelevant for effects like Hamatula Strike*. I agree that you can't enchant the hammer with keen.
*) "Whenever you damage an opponent with a piercing weapon" - you damage an opponent, with a weapon, that deals only piercing damage. I repeat, "Weapons are classified according to the type of damage they deal",if it deals piercing damage, it's classified as a piercing weapon, for that attack.