You can reinforce an enemy village to steal their wheat. Whether or not the have an army there is irrelevant.
Four things to be aware of, though:
They can send your troops back at any time, simply by using a rally point in that village (assuming they're active enough to notice your troops stationed there).
You have no way of knowing their exact crop production, nor the size of their granaries, so you will have to make guesses as to whether they have enough crops to support your troops. If they don't have enough production to support your troops, your troops will eat their reserves. If they don't have enough reserves to last until you pull them, your troops will starve. A clever player can even use their marketplace to hasten the removal of crop reserves, making it more likely that you'll starve your troops if you send more than the village can support.
Your troops will always starve before theirs. When a village doesn't have sufficient crop to feed troops stationed there, troops start dying in a specific order, starting with any troops sent by other players as reinforcements first, and ending with troops produced by that village. You will always lose every single troop you send to reinforce before any of that village's troops will start to starve.
Offensive troops are lousy at defense, and if they have offense at another village, or a friend with an army, they will likely try to kill your army in their village if they're even remotely competent. If they aren't competent, chances are decent someone else is farming them, in which case they may attack your troops accidentally.
In short, yes, you can do it, but it's probably not a good idea.