Fortresses are self-sustaining in regards to food. However, garrisoning too many troops there could cause some food problems.
As for traders, no skill is required. In fact, all it would take is a steward. You could have the steward of the realm providing the food journey to the realm being fed, and then just accept any Buy offer posted in the region being fed. Or, you could have the steward of the realm being fed travel to the realm providing the food, and then buy any Sell orders being posted. There's probably other ways to do it, too, including high-skilled traders posted somewhere in between. Or chains of lords, etc.