Yes you can, and a lot of people do. There was an article about this in the SMH last week.
https://smh.domain.com.au/real-estate-news/upwardly-mobile-20120518-1yu6i.html
The process is not simple and involves getting the approval of the owners corporation who would have to agree to an exclusive use by-law giving you use of that space because it is common property, i.e. it is owned by all the owners. Amongst other things the by-law would make it clear that you were responsible for the ongoing maintenance of that space. You would also have to pay for having exclusive use and that would be calculated by taking the cost of converting the roof space and deducting that from the assessed value of the space once it had been converted. For example if you paid $50,000 for conversion and the assessed value of the space were $100,000 then you would pay the OC $50,000. You would also have to pay to have the by-law drafted and registered.
From the point of view of most OCs they are unlikely to object to space being used that no-one else can use, provided they are satisfied that engineering requirements are met, the common property isn’t damaged etc.
You would have to draw up detailed plans, get engineer advice and put in a DA to the Council which would require approval of the OC before lodgement. A major consideration is whether it is worth the costs relative to the increased value of the apartment.