You can't just "buy" a Skycouch if you have a row of 3 seats booked - the Skycouch product that is sold in this case if you upgrade to the Skycouch (which at a guess will probably be about 75% of a single economy fare) will also include an extra seat since the Skycouch product isn't usable if there are 3 people in that row already.
Air NZ may be able to help you out if you contact them, you just need to decide if it's worth paying the extra for the product.