I'm an Italian student holding a Bachelor Degree in Foreign Languages ( however with a focus on International Politics ) and currently enrolled in a Master Degree in International Relations, focus on Transatlantic Studies. Does someone know if there are programs to get some deeper ( and certified ) legal knowledge, especially but not only, in the field of security/defense?

There is a programme at Fletcher School of Diplomacy (USA) that may interest you, it is called "Master of Arts in Law and Diplomacy". The school seems to be outstanding, however take into account that it is a two year MA.

http://fletcher.tufts.edu/MALD