Hi Andrew.
Unfortunately I don't know of anyone in the greater DC area that I would trust with my kilts. (and I can't sew either)
If it were me I'd try to send the kilt to a kilt maker (preferably the one who made it) to have alterations done. If you can't do that I'd check the local pipe bands (I know there are at least a couple) and see who they recommend.
Dee
Ferret ad astra virtus