It's possible to upgrade to a double pedal, however, the answer really depends on your current pedal. The easiest way to upgrade is to buy a 5012--the primary side of a double pedal unit and the connecting universal rod. Your existing pedal will then be used as the slave pedal in the double unit.
Before you order one, you will want to make sure that the stroke adjustment (the black, tear-drop shaped piece that sits above the spring) attached to the hex shaft is SQUARE. In other words, the end of the hex shaft that sticks out of the pedal needs to be square in shape. If not, the 5012 will not work. A 9000 single cannot be upgraded to a double pedal.