I believe they're extracted from xplrbase.viv, which you can do using the EA Graphics Editor. There you'll find the arm and leg textures for all the different skin tones, which you can edit and rename (player)_arm_tat.fsh as necessary.
I also remembered that we do actually have a tattoo tutorial:
http://www.nba-live.com/nbalivewiki/index.php/Tattoo_TutorialIt was written for NBA Live 2005, but the basic principles should be the same as far as editing the texture.