I am enjoying this exquisitely Y4L, because, to me, it is the MVP baseball of hockey games.
As far as installing season mods, you must install the original game to a short path like C:NHL 2004 and NOT a long path such as C:Program Files (x86)EA Sports NHL 2004 because the path will be far too long to install mods.
You'll want to make a copy of your NHL 2004 install files to install the 2013 mods. Install the mods to C:NHL 2004 and afterwards rename the clean version of 04 something like "Original NHL 2004" or "Clean NHL 2004" but NOT "NHL 2004 Original" as it will confuse when the system is searching for the game.
You will only be able to mod and play the version named "NHL 2004", so you can therefore "trick" your system of having multiple installs by simply naming whichever version you want to play to "NHL 2004" and the others as something else. If you ever want to play the original, simply rename it back to "NHL 2004".