I have two questions how does one add an extensions page to the main game as a sub game page. Also how to create them?
The easiest and most practical way of doing it, is through the misc. tab on the main game board. This makes it so you can keep all your categories on the same page, and not have to worry about moderating two boards.
If you want category extensions, use the link above from @Liv , but the misc. tab is usually the better route to go.
@Cheddarcravescheddar You can request a sub-game through support hub, the normal way to request any game, but you need to select game types that support a base game and put the parent game as the base game. So ROM Hack, Modification, Fangame, Prerelease, Expansion/DLC, Category Extension, Minigame/Gamemode and Server/Map are all the game types that have a base game