On the matchup screen of a Preseason game, press SQUARE to go to Preseason Options. Press L1 to Toggle the Home Team as the default stadium. Then scroll to the stadiums you want to unlock. Enter the following codes to unlock them:
Code - Stadium
CAROL - Old Carolina
SEPIA TONE - Good Ol' Days
GOVT OWNED - Galactic
5280 FEET - Old Denver
LIGHT YEAR - 2050
AUTOMOBILES - Old Detroit