I have no idea why you'd want to do that, but it could be done fairly easily without touching the textures.
1. Take the portrait you want to use.
2. Re-name it to match the PLAYERPKG of the player whose portrait you are replacing (eg plyrprts~PAGEORG.big to plyrprts~RAALLEN.big)
3. Open the file with a hex editor, such as HxD
4. Change all instances of the PLAYERPKG to match the filename (eg PAGEORG to RAALLEN). Most hex editors should have a find/replace all option, which makes this a lot easier.
5. Save the file, place it in your sgsm folder.
Alternatively, you could just make a custom portrait:
http://www.nba-live.com/nbalivewiki/ind ... t_TutorialAgain, no idea why you'd want to do this though.