Distinctions should be made.
Questions about using bundled modification tools
- "What does 'external height' field mean in the Darwinia map editor?"
- "What's a good tutorial to start making Quake 3 maps?"
These questions definitely belong here, in my opinion. They are about using tools provided with the game itself, and are in no way related to game development or design.
Questions about editing game files
- "How do I make a Dwarf Fortress skin?"
- "How do I customize the Allegiance hud?"
These questions usually belong here, in my opinion. Even when not facilitated by external graphical tools, they usually only involve changing data structures exposed by the game itself.
Questions about adding new features to a game
- "How can I add cats to IVAN?"
- "How do I make a new gun in UT3?"
These questions definitely do not belong here. Personally I would vote close, then still give a quick answer (or rather comment?) with pointers to where to ask instead.
You can add cats to IVAN -- but you must change the game's source code. This game is closed source, so you will need to ask the developers on the game forums.
You can add the tickling gun to UT3 -- but you must change the game's source code. I do not have the expertise to tell you where to change what how, but the guys at Game Development sure can. Why not ask there?