Aight..I'm pretty tired right now so I'll probably edit this post later and make it a lot clearer and stuff..but here goes:
Say you want to add ssj3 goku, but you don't want to overwrite any default models. Here's what you do.
Go into your ESF/Models/Player folder, make a new folder INSIDE of that folder, name it SSJ3Goku. Then, take your ssj3 goku model file, and regardless of the name, rename it to SSJ3Goku.mdl. Then place it inside your SSJ3Goku folder. Remember, your folder and your .mdl file names have to be EXACTLY the same. If your not sure what I mean, go into one of the default folders. You can name the folders and models anything you want.
Now, if your playing on someone elses server, to access your SSJ3Goku model, go to your console, and type model ssj3goku.
If your model turns into SSJ3Goku, then you did it correctly. If it turns into normal form default goku, you did something wrong in renaming the files or placing the files in the correct...folders, or you misspelled SSJ3Goku.
Now, if your model stays the same, then the server admin has enabled a "Locked Models" command. If this is the case, you can ask the admin to input this command into his console: "mp_lockmodels 0". Now, if he says "It says unknown command", tell him to type it CORRECTLY.
Now, if you are playing on your own server, just input that command, and after doing that, a message in the console should say "mp_lockmodels change to 0".
Remember, normal form goku is the default model, so when you are trying to change to SSJ3Goku, use a different model.
If your not sure about the model command, try using it with the default models. Type like "model gohan, ssjgohan, frieza, frieza2", etc. If you do these commands when mp_lockmodels is set to "1", then your model will not change.
If you need a deeper explanation than this, or you cannot get it to work, contact me on MSN or AIM.:]