You'd have to actually build a ladder support and cross-bar system for it. The standard hannibal is gutter mount. That being said, LenB has done this - he adapted a Urban Offroad rear ladder and used regular LR crossbars up front. Unfortunately, that results in a tubular ladder and a square tube rack, so they don't "match" exactly. The NAS defender kind of has the round-tube "look" going already with the cage up front and the tire carrier, light bar, and bumper out back - which is why almost everyone likes the look of the stock Safety Devices rack. Rovers North offered a square tube rack for the soft top that had probably the worst looking rear ladder ever.
An alternative is this solution by Trek Overland, they used a pair of square-tube ladders to good effect with the hannibal rack. You'd have to modify the second ladder for tire-carrier clearance on the NAS soft top, but at least this solution attaches to the crossmember and doesn't require drilling hte body.