The Animation Library allows you to specify the local transform to use for any bone and override the animation transforms for that bone. As an example, let's make the dwarf hold his weapon upside down.
dwarfAnimator.BonePoses["weapon"]
    // Add this to the end of the Update method
    BonePose weapon = dwarfAnimator.BonePoses["weapon"];
    weapon.CurrentController = null;
    weapon.CurrentBlendController = null;
    weapon.DefaultTransform =
        Matrix.CreateRotationX(MathHelper.Pi) * 
        Matrix.CreateRotationY(MathHelper.Pi) *
        Matrix.CreateTranslation(weapon.DefaultTransform.Translation);