Pretty sure you'd be able to implement third tiers via the trainee branches, but I'm not sure how much room there is for new data with FE8. However, FEXP is available to work with, though I dunno how well supported it is at this point - if you look into Fire Emblem Immortal Sword (FE7x) you can find info on FEXP there, as it was the original engine being used to create the project.