conversion intended as: "i saw you and now you are mine!" (aoe sheeps convertsion style) for something not consumable and that provides benefit to a player seems hard to achieve if the unit in question is randomly generated somewhere in the map and it has to be brought in own base (indeed there will be many back and forth in order to re convert the unit and perhaps a capture-like system would work) unless there isn't something like priests in aoe who, once captured a relic, they bring it to a temple (a woman carrying a sheep to his base looks quite fun).
@Servo Farms can infinitely produce food but in my opinion the most important aspect of managing own economy is to minimize the number of units involved into economy and focus, perhaps latter in the game, into the military part of the game (sieges require pop). Thereby the amount of food you need for late game is lower than the amount of food provided by the grain fields you have to build during the game in order to sustain a constant units production until the population cap (unless you plan to train cavalry champion which is an extremely food hungry unit). Perhaps it also depends on the population cap.