Giving a ghost a Young again potion would only make them go back to being Young Again. It will not reverse them being a ghost like Ambrosia. So if you want them to be a ghost but yet then use the potion. If you want them to be alive again, you have to do ambrosia.
Also if there is a particular ghost you want, you can control them if you do the special opportunity from the science lab. If you already control them, then I don't understand why you wanted to know how to get them to eat it.