Unanimity is required. Converting a common area into a private area for private use is not an ordinary decision: it requires the agreement of all the co-owners (art. 22), not just a majority. The law classifies this operation among the most significant decisions, on the same level as constructing a new building, adding a story, excavating, or completely demolishing the building.
There is an important limit: this conversion is prohibited when it concerns the common areas listed in article 4 (for example the major structural works, the foundations, the load-bearing walls, the façade, the stairs, entrances, and corridors intended for common use, the common elevators). A hallway intended for common use falls precisely within these areas, which makes the operation very difficult, if not impossible.
Also bear in mind: the common areas and their accessory rights cannot be subject to division, seizure, or transfer independently of the private areas (art. 7). In other words, no one may appropriate a common area for themselves alone. If your neighbor acts without this unanimity, you may bring the matter before the court of first instance of the place where the co-ownership is located, which has jurisdiction over any dispute relating to this law (art. 59 undecies).
This is general legal information, not legal advice. For advice on your specific situation, consult a lawyer admitted to the Bar in Morocco.