If the body corporate says no to the DA? Whats next step?

Not much.

Generally speaking, most units are of such a size that 'adding' a bedroom means you will have two sub-standard sized rooms and Body Corporate, acting in the interests of all other owners in the complex, do not want to devalue their assets by having a less than optimal solution in the building.

You are not adding any additional GLA however town planning regs would have dictated the mix of 1. 2 & 3 bedroom units, parking requirements, sewer/water, power etc. So even if you overcome the first hurdle, you probably will not get it past council easily and you may have to pay S94 Contributions for the additional dwelling/room. An additional bedroom may impose an additional parking space or visitor space requirement on the complex and would not be permissible or warranted.