No, I can't get you Stamps.
The rules say that when playing with 5 players you simply follow all base game rules. Does this mean that you still only play with 5 Spike magazine cards? We tend to give each player (in a 4 player game) 1 turn with the coin and then randomly assign it for the Blood Bowl itself - this wouldn't lend itself well to 5 players.
I don't see why the number of Spike Magazine cards (rounds) would change. You use 5 cards for 2, 3, and 4 players, so why not 5?