I don't think that's possible to convert a mono MP3 to stereo MP3 in Spectrum Lab. I've searched or this application and found out that it can only analyze the frequency and spectrum, but nothing on the conversion part.
You can use other application for this operation. For example, install Audacity, load the track, then go to Edit > Duplicate. From the arrow next to Audio Track, select Left Track, and for the next one select Right Track, then go back to first layer, click the arrow again and choose Make Stereo Track. Save the file and you now have a Stereo track.
Note: You might need LAME MP3 in order to save as MP3. Download: http://lame.buanzo.org/