Get mob's default NBT data

I wan't to place custom spawners including NBT tags for example: spawner set to skeleton and skeleton type is set to 1 for wither skeletons

And is there a way to get the default NBTTagCompound of mobs?

Like:

NBTTagCompound nbt = new NBTTagCompound();
EntitySkeleton.writeToNbt(nbt);
// nbt should now be filled with ALL default nbt tags from the Skeleton Entity INCLUDING the SkeletonType tag!

Crafting/placing all types of spawners is working but I am stuck on how can I change specific NBT tags (skeleton type, zombie villager, etc.)