These are the files that are hidden inside the SP2.glm file, which is simply a (custom?) WAD file
(in fact, the class that jrcouture gave me was called "GWadManager"), in their original folder structure.
While as the time of this document being written there are no ways to modify or create GLMs (my tool can only extract and navigate the files), that is unnecessary since the game engine prioritizes files that are already on the disk if there are any.
In simple terms, SP2's file manager works like this when the engine requests a file:
1. Find a file in the disk (regular filesystem, the mod's folder). If found, use it
2. If, and only if, this file is not found in the disk, find that file on the GLM and use that.
Note that this theory is based purely off observation, it's unknown if the file manager actually tries to find the file in both places or just stops if it's found somewhere, I don't work at GolemLabs so I can't say for sure.
Mods such as OBM and more recently Age Of Discovery (made after this was released) take advantage of this by providing their own sound, texture and other files to enhance the experience.
In order to replace a file that already exists in the .glm and have SP2 recognize it, one must, following the original folder structure place their new file accordingly. For example, if I wanted to replace the ocean battle overview texture which is located in \data\Combat_Overview\Scenes folder, I'd go to the directory for SP2 or whatever mod (where client.dll and server.dll are located) and mimic the same path, which would end up looking like this: (portion in bold describes the relative directory)
C:\Program Files (x86)\Steam\steamapps\common\SuperPower 2\MODS\SP2\data\Combat_Overview\Scenes
and place my ocean.dds in that folder, where the game would find the file and load it up.
This opens up a lot of potential, as any file can be replaced. Happy modding!
Special thanks to Jean-René Couture for making this possible