The Animation Library allows you to specify the local transform to use for any bone. As an example, let's make the dwarf hold his weapon upside down.
dwarfAnimator.BonePoses["weapon"]
    BonePose weapon = dwarfAnimator.BonePoses["weapon"];
    weapon.CurrentAnimation = null;
    weapon.CurrentBlendAnimation = null;
    weapon.DefaultTransform =
        Matrix.CreateRotationX(MathHelper.Pi) * 
        Matrix.CreateRotationY(MathHelper.Pi) *
        Matrix.CreateTranslation(weapon.DefaultTransform.Translation);