From: eastern CT
No. The server lets you play the selected game with the countries pre-assigned by the challenger as player 1 and player 2. If you accept the challenge, you have to play as player 1. I know this because I accepted a PBEM challenge, and when it came on player 1 was listed as Britian, and player 2 was listed as Germany, Italy, USSR, and USA!? I guessed the challenger had made an error setting up the scenario, but try as I might, I could not change the setup. I ended up withdrawing from the game, without ever playing a turn.