Itunes can import any MP3, AAC or Apple lossless file into it's own library just fine, unless it has some sort of DRM encryption on it. As most songs on Kazaa, Limewire and elsewhere are MP3, all you need to do is import them into iTunes and then sync with the iPod. There is no way for iTunes to know whether the song is pirated or not. Note that just copying them to the iTunes music folder directly doesn't work as iTunes still won't know about them.
goates