Sir Alex Ferguson might not be signing Usain Bolt, but he is ready to let the sprinting superstar loose on Real Madrid.A lifelong Manchester United fan, Bolt has already been to Old Trafford since his triple gold medal-winning exploits, telling the crowd ahead of last month's 3-2 win over Fulham that they needed to persuade Ferguson to sign him.That is clearly not going to happen. However, Ferguson has raised the possibility of Bolt fulfilling his dream in a rather different way."He's been to Carrington a couple of times and he did help Ronaldo with his sprinting," Ferguson told Inside United. "He's a bit of a character and a big United fan."But it's interesting he says he would like to play in a charity game. It could be brilliant, and next year, when we play Real Madrid's Legends again, there could be opportunities to bring him in and see how he does."