Consent is required
Partial subletting (a single room) is possible only if the contract doesn't forbid it and with the landlord's consent.
Many student contracts expressly forbid subletting: always read the clause before proposing a flatmate.
Putting it in writing
The landlord's consent must be obtained in writing, and the sublet itself must be formalised with a contract and registered.
The sublet contract should still be drawn up with the help of a professional (lawyer or tax assistance centre) and registered with the Revenue Agency.
The risks of illegal subletting
Subletting without consent can lead to termination of the contract and loss of the deposit.
Whoever takes an irregular sublet is also exposed: no protection and possible sudden eviction.
Informational material by casaperstudenti.it: it is not legal advice. For your specific case, consult a qualified professional.