While it would not necessarily be out of the question, it could just about be. The roof of a block of flats is generally common property. As such all owners have an interest in it. For you do an extension into this area you would need to be granted a special privilege over the common property (an unopposed resolution in the ACT, a special resolution in NSW, I think). There would need to be by laws created to make you responsible for maintenance and so on. Both you and the owners corporation would want good legal advice.
Alternatively, the scheme could be resurveyed with the area made part of your unit area and unit entitlements reassigned. This is also a major undertaking requiring a high level of support from owners.