September Update
Posted one month ago
Published maps can now set up their own Stats and Achievements. How they're used is really going to be down to the map maker, and the game that the map is targeting.

To add a stat, you just use the Services stuff in ActionGraph - so you can tie it to any logic that you want.
And to unlock an achievement manually, same deal.
Again, there's no real way to use this functionality in Hammer yet. Our intention is to make it all work in Scenes, and then make Hammer able to use Scenes - so it's all kind of the same system.