As far as I can tell, it's not ever updated. In my game, I've added and modified some outfits in Jennifer's wardrobe, but if I import an outfit and select Mom_Wardrobe (which I'm assuming then pulls from the XML file), it just shows the default outfits she started with. I'm pretty sure those XML files are meant to be a hardcoded list of what outfits the character starts with. Hence, if you modify that file from outside the game, it will change what the characters starts with for all your subsequent new games. But again, I haven't tried it myself.