Actualy, the first thing you must figure out is where the shotgun fire sound is located in the GCF. So use GCF Scape to do so. Obviously, sounds are located in the file source sounds.gcf, so open that. Then, find the weapons folder. Its located in /root/hl2/sound/weapons/shotgun. There are 2 fire sounds in there: shotgun_fire6.wav and shotgun_fire7.wav.
Now that you where the fire sounds are located, goto your mod dir and create the exact same folders as in the GCF, like this:
half-life 2/hl2/sounds/weapons/shotgun.
Put your custom fire sounds in the shotgun folder and you're done.