You may be able to find a double crank that fits on your existing bottom bracket. Make sure that the chainline is correct - otherwise, you may not be able to get your front derailleur to shift properly.
If you can find such a crank, you may be able to get the rest of your setup to work.
You shouldn't need to replace your rear derailleur, and you should be able to adjust your front derailleur to shift a double properly.
http://www.velonews.com/tech/report/...es/9053.0.html