You can easily convert MP3 songs to Ogg Vorbis audio format with programs like Any Audio Converter or Freemake Audio Converter or any other listed below.
Check out Helium Music Manager. It is the best music management software I know and it also can convert many audio formats to ogg. Helium Music Manager is available also in free version.
MediaMonkey is similar and also very good free music manager that contains CD Ripper & Audio/Video Converter: Encode MP3, OGG, FLAC, AVI, etc.
On Mac platform try to search the App store for mp3 to ogg converter (use it as search string) to find free or commercial converters.