Can I connect PC's serial port 'extension' cable or a "PC Com Port null cable" to a "Madcatz Microcon MC2 Xbox 360" wheel to extend the length of a cable for the pedals?
If not, how can I extend this wheel's pedal cable?
I can't find a photo of the connector used, but if they used a standard DB9 serial connector to join the pedals to the wheel, it should be possible to extend it with a straight-through male to female serial cable. The male half of this type of connector looks like:
You do not want a "null modem" cable, as some of the pins are switched in that type of cable. They tend to be female-to-female as well.
The only other concern is signal integrity, which is difficult to estimate. Pick the shortest length of cable that meets your needs to get the best result. If the communication between the two halves of the devices is particularly poor, extending it might make it worse or cause it to stop functioning. The RS232C spec gives a maximum length of 10 feet, although there's no guarantee that MadCatz followed the spec.