That works only on the Windows version of iTunes. For Macs, try EasyWMA. Of course, it's better to re-rip the tracks if possible, to ensure the best quality. 🙂
And if the original is a DRM encoded WMA, you are out of luck. The only thing you can do is burn it to an audio CD (if the Windows music player/online store software allows it) and re-rip in iTunes
That works only on the Windows version of iTunes. For Macs, try EasyWMA. Of course, it's better to re-rip the tracks if possible, to ensure the best quality. 🙂