sure.
You get the model from the silly zip file that you downloaded. Then you take the .mdl file, and put it in your esf/models/player folder, BUT you must re-name the model, to whichever character you want it to replace. Example : I found a model, called ssj3goku. I want this to be the model i see when normal goku does his transformation. I go to the ssjgoku folder, rename the .mdl file that i just downloaded to the exact name of the one i am replacing. then put the new .mdl where the old one is. This will replace the model. *NOTE* BACKUP YOUR ORIGIONAL MODELS FIRST!!! this is a must. i have 2 copies of HL on my HD. one i use, one is all backups, so i can replace any customizations i do.