Well you cannot add more models. You can replace the default models with other ones. Let's say you want to replace a goku model with a ssj3goku you downloaded.Rename ssj3goku.mdl to goku.mdl and move it to esf/models/player/goku and replace your old goku.mdl with the new one. But that's just a model replacement so it won't have anything new. ESF v1.3 will have more transformations for each character.