Is the USB connection really the only one, with not even an analogue audio input socket that can be connected to a portable player? If so, which kind of USB: the kind that is the external interface for a storage device and expects to be connected to a computer, or the kind that expects to be connected to a storage device such as a USB stick? These aspects determine the mechanism for transferring the tracks.

Whatever the input arrangement, other formats such as WAV, FLAC, etc ought to be accepted as well as MP3 so, depending where the tracks are coming from, you probably don't need to recode them but only to transfer them. If you're taking them from CDs you do need software that can "rip" them. (To me that word suggests "rip off", implying illegal copying, and indeed copying from CD to some other medium may be technically illegal, though in practice you're unlikely to find yourself in court for it.)

There might be yet other possibilities: perhaps the car would accept an external CD drive with a USB connection.

Anyway I trust you have already RTFM.