Grilled Cheese is a 'secret' aspiration, and as such, it does not appear as a selectable option when a Sim is created, grows into a Teen, or begins his/her junior year at university. Once FreeTime is installed, though, Grilled Cheese can be chosen as a secondary aspiration. It is also available in the sim modder aspiration menu. Another way would be through Insimenator or Simblender. Grilled Cheese Sims can change their aspiration just like any other Sims (by reaching junior year at university or using the ReNuYuSenso Orb properly).