If the wma files are already in your iTunes, then find them. Go to your iTunes Edit menu -> Preferences -> Import settings. Choose either mp3 or mp4 (AAC). I'd recommend VBR via using custom settings. Choose your preferred bitrate, click VBR option, then press Ok, Ok. Close out from Preferences. Select the WMA files in your iTunes library. Then go to Advanced menu -> choose Create .... version, be that mp3/aac, etc. That's it.

To find your WMA files, you can click on the top of the Kind heading menu across the top of the Library window so it arranges the library via file-type. Then scroll down bottom to WMA. Else double-click the Kind menu heading & it will put WMA on top of list.