hmmm... id say SP would work just the same as MP.
you figure out what skin he is wearing (the hard bit) copy those TGA files and change them to whatever you want and save into a PK3.
easy way would be to keep the same player model (ranger most likely) and just have a PK3 with the TGA files in it. hard way would be to do a full skin job with all the TIKs where u can change models but thats beyond my brain and im hungover so leave me alone now.
Somewhere in the SP code, the game is telling it to use a specific skin, whatever it is (I haven't used 3rd person). I'm guessing it's the Allied Ranger. Anyways, you would have to find whatever that value may be, and change it to a value of another uniform. Either that, or alter the original uniform's files like r3mix said, and just replace the old ones with the ones you made.
to clarify this, yes you can change skins in SP. i have run it in 3rd Person and have found the command. however im not at home right now, so i wouldnt be able to tell/show you that this works. when i get a chance i will post a pic from my comp.