First try AlanH, his thread is currently active on the X-Kilt. I believe some members of the forum have already added pockets. A popular pocket is one applied in the upper center of the under apron to hold goodies as well as to weigh it down for privacy whilst seated. In one thread that I can not find right away, a poster installed two front slash pockets. between the first and second pleat on each side of the kilt, as well as the under apron pocket to hold a vast quantity of supplies.
For the under apron pocket idea, check out WizardofBC (Freedom Kilts) above as it is a feature on the kilts Steve makes.