You would have to join as a S/O and then bid for a promotion accordingly. There are no DEFOs or DECs to mainline.
The only time time I recall it happening was post Ansett collapse in 2001 and I believe that was for a specified period in rank only.
If if you haven’t applied yet though - sounds like you haven’t - you might be waiting a while from here. They’ve just really started processing another 500 applicants after rejecting another few hundred.