just download,lets say, ssj4goku.mdl for goku.
goto his folder, paste ssj4goku.mdl into esf\models\players\goku and rename ssj4goku.mdl to whatever model file there is in that folder. in this case you'd rename it to goku.mdl. if you wanted to have ssj4goku.mdl for ssj goku, then in the folder you'd re-name it to ssjgoku.mdl