Select a pitch, then press Down + Square while more than one pitcher is in the bullpen. A message requesting another pitcher will appear at the bottom of the screen.

Go to the team that has the desired player. Locate the bar that with the word "Computer" and change it to "Human". Propose a trade with your regular team. Return to your team and select "Trades Pending". Accept the trade, then return to the team where the player originated and select "Trades Pending". Select "OK" and change the team back to "Computer".