deconstruct the wagon, channel a 4X4 area around where the wagon was, set that ditch as a meeting place, everyone goes into there, remove ramps, construct ceiling to prevent rain/undead/clouds from entering, dig.
surviving migrants will be hard to come across, and a zombie apocalypse isn't out of the question. one zombie kills a guy, they're ressed, they kill some more, those guys are ressed until the surface is a mass of living limbs.
do not butcher ANYTHING! the hair and skin produced can come to life, and no matter how many times you kill them, they WILL come back. only, however, if you embark on an undead evil area. you can tell if it's one of those, because the non-cave wildlife will be undead.
Cool, but will ANYTHING that dies come back? or only if a cloud hits it? and whats the deal for underground caverns too?
If your biome is reanimating, any full bodies, hands, and heads (things with GRASP and such) will reanimate forever no matter how many times you kill them, unless you destroy them through magma/bridge-smashing/processing them. This effect extends to all parts of your map that is a part of the reanimating biome - no matter how far into the caverns you dig, if the surface above reanimates, the underground will too. I once accidentally started a zombie apocalypse in a cavern this way by dumping two dead bodies down there - they reanimated endlessly, slowly killing off the animal people and even a forgotten beast. All casualties would then also raise up, adding to the horde.
You
can butcher things, but you have to be fast and efficient about it. Have a talented butcher, tanner, and spinner, disable all of their jobs besides those, and don't have a refuse stockpile to put skin and hair in. Make sure all workshops are next to each other. Butcher the animal. The tanner and spinner should immediately grab the skin and hair, respectively, and start working on them. Once tanned/spun into thread, the hair and skin will be gone and not reanimate. Of course, if your dwarves get it done before it reanimates is the big question, so if your dwarves aren't very good at the job it's best you don't butcher, or only do so with your military stationed around the workshops. Therefore I wouldn't recommend butchering until you have a sizable amount of dwarves with some sort of militia going on.
An upside is that if it does reanimate and your military chops it up, every new bit of skin/hair can be processed individually, so a clever player could maximize upon this and get a lot of leather and thread from one butchered animal.