Is it possible to change the pocket dimension without changing core files? If so, how do I do it? And if not which magicBoxRooms.xml do I replace; the one in Dungeons of Dredmor/Game, the one in Dungeons of Dredmor/expansion3/game, or both?
Which magicBoxRooms.xml do I need to modify then? The one in Dungeons of Dredmor/Game, the one in Dungeons of Dredmor/expansion3/game, or both?
Good question. You don't need the expansions to access it, but I don't know if it's any different when you do have it. I'd say try one (the base one) and if it doesn't change it for you, try the other.
I know it's sort of late and that it doesn't really answer your question directly, but since it's been months since I tried to take a "deeper" look at the room definition files, I'll just say this. There is a mod which changes this particular room, and if you want to learn what to modify I suggest you to take a look at it. It's this mod, by the way: http://community.gaslampgames.com/threads/core-expanded-pocket-dimention.5199/