Before beginning a new game, go to the Game Setup screen (the one with two helmets at the top of the screen), highlight either the "Home" or "Visiting Team" option and press the following codes to be the expansion teams :

For the Jacksonville Jaguars, press: L, R, L, R, A

For the Carolina Panthers, press: L, R, L, R, Y

NOTE:

They will both have 99 for all of their team statistics.
These codes can only be used in "Exhibition" mode.