The National Football Stadium was built by Populous (the same people who built Suncorp) and was always built so it could be extended easily to 25,000 seats.
The original plan was for the additional capacity to be added prior to the RLWC, however it was de-prioritised.
The NFS already has corporate facilities for 500 as well.
Not sure how they are fitting 20K into Sir John Guise Stadium. The venue has less seats and room to expand than the NFS. It was built by Fletchers Morobe Construction and wasn't designed for expansion as it is part of a greater sporting complex and NFS was set to be the primary Stadium for Port Moresby.