You could try doing it as a faction-wide food bonus, like the grocer line of town buildings has. If you wanted it to be half of the village's food, you would make it so that you have some structure (say, Feudal Manor, or some such) that produces X food per grain locally, and X faction food per grain. This, unfortunately, would apply to everything in your faction, though, so it might not be quite what you want. If you wanted to add a money bonus, then you'd either add a percentage bonus to money production or a flat money bonus (or both, if you wanted).
Alternatively, you might see if there's a way to create a trade caravan-like unit that goes between your town and a neighboring town which provides X food per grain, or Y grain, or something along those lines, but I have do not believe this is possible.