If you want to be technical, Episodes 2-5 are all DLC.
But anyway, I kind of doubt Telltale will ever deal with DLC (in the conventional sense) due to the episodic model. They have confirmed a second season, though, so there's that.
As for mods...you can't really mod an adventure game. At least not in the same sense that you could mod, say, an FPS.
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