Little more detailed information:
All the model files are named *.MDL (stands for Model). The character models are located in the following folder: "Steam\SteamApps\youremail\half-life\esf\models\player"
In all those folders in 1 MDL file located, such as in the Goku folder you'll see "Goku.MDL". When you download new models, they are of course as well MDL files. If you want to use any of those models, simple overwrite the new model file with the original one.
For example, you downloaded a model named gokussj4.mdl, and you want to replace the super saiyan goku. Then go to the folder "ssjgoku", you will see in it "ssjgoku.mdl", remove that ssjgoku.mdl, and add the gokussj4.mdl in the folder. THEN rename gokussj4 to ssjgoku. And done you are