From the 'Secret Codes' Menu, enter any of the following codes to unlock all players on that team:

Unlocked Team Code
Giants Biggiants
49ers Goldniners
Jets Greenjets
Bears Brownbears
Lions Silverlions
Bills Bluebills
Packers Yellowpacks
Broncos Buckingbroncos
Panthers Bigcats
Chargers Bluechargerst
Patriots Redpats
Chiefs Redchiefs
Raiders Silverraids
Colts Whitecolts
Rams Goldramst
Cowboys Bluecowboys
Redskins Redindianst
Dolphins Aquafins
Saints Marchingins
Eagles Greeneagles
Steelers Blacksteels
Falcons Blackfalcons
Vikings Conquerd