How do you make an item to act like a farm?

Just as the topic says.

Is it even possible?
If so, is there a way to make the item work on regular units?

How do you mean "like a farm" and "on regular units?"

If you mean you want it to provide food, you could trigger it so when one is picked up it "Player - Add property"s to the owner of the manipulating hero a given amount of food, and when one is dropped it "Player - Add property"s a negative amount...