Studio One let's you Transform an Instrument track to an audio track and back, so I'm not sure why you are concerned about losing your MIDI notes.
Also, there is no need to convert your Instrument track into an audio track in order to export it as an MP3 (unless the tracks are using an external MIDI module/keyboard).
Open the Song menu. Choose Export Mixdown and choose MP3 as the Format. Set the song range selection from one of the options on the right side of the dialog.