According to the product comparison chart, Studio One prime (the free version) includes both the Content Browser, which is how you browse your computer for mp3 files that you want to drag and drop onto an audio track, and it includes mp3 decoding support. So there is no technical limitation. You should be able to browse your computer for mp3 files that you can then drag and drop onto an audio track.
If you cannot see the Content Browser, go to the menu bar at the top, click on View, then click on Browser, that will open the Content Browser on the right side of your screen.
