LUIS, it may not be clear what Jeff means by 3 wires (there would have to be 4 plus grounds if present). If the power for the receptacle comes to the switch box with a switched leg going down to the receptacle, then you can do what you want, but in the more likely case where power goes to the receptacle box with a switch leg running up to the switch (may be two blacks, or a black and white depending on the wiring method) then you are out of luck. In the latter case, you will need to pickup power somewhere and run a new switch leg down to the switch box. You can then use the existing switch, or install an additional one. When you install the fan, be sure to use an electrical box specificly designed and listed for fans; they are readily available. Using as standard box would be aginst code.