Using WiFi adapter to capture different wireless traffic
If it does use the same frequenzy as a wifi card it should be possible depending on your card, skills, and willingness to invest time ;). It might be easier and quicker to go with something else though, look at the path Tim took to make his diy-spy for instance. 

Using a device like a wifi spy should answer if it is there or not, Tim made his own diy-spy based on a cheeper device and has published the source code he used. Basing this device on a wifi card should be possible if it supports digital RSSI reporting or not. Starting of at this code it should be possible to find the signal you are intereseted in and working with that.

Another "simple" way to find out what frequency your keyboard uses would be to check it using a Frequency Counter (either borrow from a hackerspace, a friend, a ham or buy one [there are cheap options on for instance ebay]). The reason I say this is that some [I have seen] uses 2.4 GHz, and some 27MHz; and I would be highly impressed if you are able to read 27MHz using a hacked wifi driver ;)

You might wish to check out KeyKerki


  This opensource hardware and software project enables every person to
  verify the security level of their own keyboard transmissions, and/or
  demonstrate the sniffing attacks (for educational purpose only).


They have done both 27MHz and 2.4GHz but this is a stand alone solution, not using a computer card.