To get any player in a trade without the other team rejecting, in the franchise mode, create a player as a quarterback and move his attributes until you get to 99 points. After that, put his injury and stamina down to 0. This will not change the attribute point level. Sign him as a free agent then trade him to any team for anybody you want. When you play the team he is playing for, after a couple of plays he will either become injured or will be taken out of the game for his weak stamina. No team will refuse your trade and you will have any player you want on your team.
During franchise mode you can trade for any player you want without having that trade turned down. Go into the create a player menu and create a QB that is all 99s. Sign that player to your team then trade him to another team that has a player that you want. To make the trick even better, you can add another player from their team to the trade that is in the 80s; this will be a 2-for-1 trade. When the trade is complete, exit out to the franchise main menu and enter back into the create a player menu. Return to the QB's attributes and edit them all the way down to 0s and then go back into the trade screen. Take the second player in the deal and trade him back to the team for the created QB. You can then start the whole process over by going into the create a player menu and editing his attributes all the way up again.
If you want to get anybody (including 99 rated players), get Corey Dillon on your team. You can trade him for anyone. The only two players that you can get together are Charles Woodson (Oakland) and Sebastian Janikowski (Oakland). Also, play or simulate all but one week. Choose "Standings" to learn which team is worst and trade with Corey Dillon for their first, second, and third round picks.
Make plenty of 99 stat players. Start a franchise and put on no salary cap. Trade them for the players you want then set their stats back to 12.