ok, goto your models directory.
which is
c:\sierra\half-life\esf\models by default
and then create a folder named, ssj3goku
copy and paste the model file in it but also name it ssj3goku.mdl
after your done. start a game, or goto a game which has the modellock off.
but if your start a game, you can jsut type mp_lockmodels 0
or goto advanced when your in the map selection, on page two uncheck the model lock.
after youve created your game.
type in the console
model ssj3goku
and you should look like ssj3goku, but you will not have his stats.