This assumes you have the rights to use the music you are downloading in your video.
If available download the higher res version with the DRM-free music (this does not give you the rights to use download music).
Or you can just burn the music to a cd and reimport the songs, they are now ready to convert .
In iTunes pref set importing to AIFF and set custom encode – 48800/16 Bit/stereo.
Select the song(s) in itunes and under Advanced there will be a convert selection to AIFF. These can now be brought into FCP.