To keep your fancy and expensive gloves dry through snowball fights, all you need is a pair of disposable gloves. Any disposable gloves made of rubber, silicon or whatever other waterproof stuff will do. I use disposable kitchen gloves (stole it from my wife's kitchen supplies ;)) Would be better for our purpose to use larger size disposable gloves than your usual glove size.
First you put your nice dry warm gloves on. Then put disposable gloves over your gloved hands. Ta-da! All set and ready to snowball fight! Remember to properly dispose of used gloves - put them to the trash.
Have fun and happy snowball fights :)