Your authority is right: you can endorse a new type rating on your AML once you've completed a theoretical and a practical training and related examinations on a Part 147 approved organization. A course can be approved by the authority to a Part 145 only when there's no availability of a proper one from a Part 147, that means you'll find extremely difficult, if not outside of the regulation, to have your training approved outside the one offered by Agusta or other schools.
But you're lucky because, as you've already another type rating onto your AML, you can save the on the job training that is the period, required in addition to the practical training, to be completed at a Part 145 on a real maintenance environment for the helicopter that you need to endorse into your AML. Such OJT is only required for the first type on the category/subcategory.
Good luck