Successfully complete Story mode with the Game Boy Advance game Eyeshield 21: Devilbats Devildays in the cartridge slot to unlock Kengo Mizumachi and Shun Kakei. To use Kyoshin Poseidon uniforms, hold L while choosing an edit team with Shun and Kengo in it.
NASA Aliens team
Successfully complete Story mode, then the NASA Aliens team will challenge you. Defeat them to unlock them as a selectable team.