Transcoding results in degraded sound quality, but if you're not much of a connoisseur (or connoisseuse), you probably won't notice.
To do so, just change your import settings in iTunes to AAC, then select the songs you want to convert and go to Advanced -> Create AAC Version. Once it's done, you can just drag and drop the songs from iTunes into a folder to copy them there. You'll probably want to delete the copies you created, so create a smart playlist with only the songs created today, select them, and press option+delete, confirming that you want to move the files to the trash.