Do you mean
.oxm? That's just a version of .xm module music with OGG compressed samples.
This explains how to convert modules to OXM.
But since you mentioned want to convert .mp3 files, perhaps you actually mean
.ogg. In that case, it doesn't really make sense to do a conversion. Verge can already play mp3 music, so it's not really much benefit to convert mp3 samples. You're jumping from one lossy audio codec to another lossy audio codec, and it's pointless to trade off even more audio quality for a little extra compression. Then again, maybe it's worth it, I haven't actually compared quality or anything of the an mp3 versus its converted OGG Vorbis format.
However! It is useful to convert .wav files into .ogg. For that I recommend
OggDrop, which lets you drag and drop wav samples and have them pooped out as compressed audio.