Hi Mauro,
I've just updated the MB Custom Post Type on Github to support moving the CPT to be a sub-menu of another existing menu. Please try it and let me know how it goes. The new version will be available on wordpress.org soon.
https://github.com/wpmetabox/mb-custom-post-type/
Updated: New version has been released. Please update and let me know what you think 🙂