i believe you need to use rpgmaker MV to do that, adding new outfits isn t that hard, having the correct picture displayed while the outfit is equipped then would just require replicating what the game creator did with other outfit with the new one.
Biggest problem is, the creator being japanese, he probably wrote everything in jap, so knowing which script does what is going to be a hell. You could probably find what scripts calls up which pic to find corresponding scripts and event but even then....good luck with that.