You have to extract the Kyle model and assets from the assets0.pk3 file and put, then change all the Kyle images and info to yours, then turn it into a .pk3 file and put in in your 'base' directory.
For example, if I wanted to change Kyle in single player to a model of myself, I would have to extract the following folders from 'assets0.pk3' - Models, Player, Kyle. And all of the files in these folders. I would then change all the files in the Kyle folder to my players files (Images and stuff). Keep the folder name as Kyle and then put all of these folders and files into a new .pk3 file (Models, player, Kyle) and put this new .pk3 file into your base directory.
You can extract and create .pk3 files using Winzip or Winace (Go through the usuall zipping procedure and change the .zip extension to .pk3 manually by typing it after your file name).
There is a possibility I have missed something out here, but try it, if it doesn't work I'm sure someone will tell you the real deal and flame me to hell. LoL.