I've said it before, I'll say it again as I will probably end up saying it forever now.
There is absolutely no way to add GGPO onto a game engine if the engine cannot support it. When building a video game, one must create a gaming engine. It's exactly as it sounds, like a car engine. It makes the car move. You tack things onto the engine when you want to add things into your game. With a software engine like a video game, there is only so much room to work with and things you can add to it. Now think about everything going on in current gen fighting games. Every possible thing you can do in KoF XII. This is allowed by the game engine. When SNKPlaymore built this engine, they did so with certain things in mind. They built it to be able to handle the gameplay mechanics and what not for KoF XII (Yes I said XII as in 12). They took into account wanting online play for their home console releases of KoF XII and with that in mind, they allowed a certain amount of room within the game engine for a specific netcode. This being GSS. Now after the failure of XII, they tweaked everything inside the game engine around to create XIII. Japanese companies more often than not, build their own game engines when constructing a game. However, this would cost a very large amount of money to do for EVERY SINGLE TITLE. As such, what do you think happens? The game engine is recycled. So the same game engine is being used in KoF XIII as XII. This is probably because when designing the new sprites and engine, they probably did so with a long-term plan in mind. Basically, because SNKP did not take into account good netplay, it was impossible for them to use anything much different than GSS due to that is what the game engine was designed around to use. There's no more room in the engine for GGPO. They goofed.
Before anyone says it: GAMEPLAY MECHANICS ARE NOT THE SAME THING AS A GAME ENGINE. PLEASE DO NOT TRY AND TELL ME XII AND XIII HAVE DIFFERENT ENGINES BASED ON THAT! THE ONLY WAY TO KNOW IS TO OPEN UP SAID GAME WITH THE SOURCE CODE AND TAKE A LOOK INSIDE
I had the above issue months back with different game around here, so forgive me if anyone takes offense to it. It's just the sad truth. Looking at the two games, and based on the lack of funding XII brought them, it seems most likely they had no choice but to use the same engine they had planned to do so. A game engine can allow people to do virtually anything so tweaking gameplay mechanics to make XII play like XIII is totally possible as well as gameplay balancing. But tacking on a completely new, process heavy, netcode? Not possible without rebuilding the engine and that is a LOT of money. Probably costs the most out of everything when it comes to game design, especially if you want it to do something complex like use GGPO (which is something Japanese developers have little to no experience in coding with). The problems plaguing KoF XIII is the same reason SSFIV: AE, UMvC3, etc don't use GGPO. It's too little too late without building the game from the ground up now.
The best thing for all of us to do in the entire FGC, is let these companies know and hound them over and over that WE WANT GGPO. You have to keep telling them so they will put it in the next game they build. As it stands, the Japanese industry does not take online serious. According to Seth Killian, Capcom LITERALLY thinks as community manager, he is not telling them what is really important to Americans. They flat out do not believe him about GGPO. The American office does, but the Japanese developers do not. This is Japanese mentality on online gaming when it comes to fighters. They look at it from offline/arcade experience with online being a fun little casual thing. That is how they see it. They literally do not think the rest of the world cares so much about it. They also do not fathom just how large countries like the USA are, or how our internet is. So when they put GSS in, they think "It'll be okay" because they all have the best internet in the world and their country is about the size of the American east coast. Granted, all this is based on knowledge about Capcom but this is also just how the Japanese industry thinks. At the end of the day, they think about making games for Japan first. The only way to convince them is to tell them otherwise that "Hey, we REALLY want this it will make us buy your games more and tell others to!" They don't think it will make a difference in sales.