Yes it is possible. But………….
The roof cavity would no doubt be common property unless stipulated otherwise. Not sure about what would actually involved exactly. Exclusive use bylaw, strata lalwyers, engineers reports, council permissions, plans drawn up, increased levies due to increased area of unit, payment to OC for use of this area, purchase of this area from OC. These things are just off the top of my head.
Even to use this space as just a storage area I think would involve most of the above, let alone a living space.