Well, you should add the code:
public void onDeath(DamageSource par1DamageSource)
{
if (!this.worldObj.isRemote)
{
EntityZombie entityzombie = new EntityZombie(this.worldObj);
entityzombie.setLocationAndAngles(this.posX, this.posY, this.posZ, this.rotationYaw, this.rotationPitch);
this.worldObj.spawnEntityInWorld(entityzombie);
this.setDead();
}
}
Haven't tried it myself, but this should work. If it doesn't look into zombie meta data.