Select Exhibition or Season Mode, then pick any team. On the
User Records screen, move the cursor under "none" and press C,
then enter a name using the d-pad. When you're finished, press C again.
More Secret Teams
At the screen where you choose teams, press x+y simultaneously.
This randomly chooses teams. Keep on pressing these two buttons
until a special team comes up such as Tiburon, All time
50s, 60s, 70s, 80s, All time(which is the best from all years from all
teams) and The EA Sports team which is rated 100 in all fields.
After starting a game, enter Tiburon as a user/player name to gain
access to all the secret teams. After entering Tiburon's name,
press B repeatedly until you return to the Team Select screen.