It is worth noting for anyone else who has a similar question and comes across this thread, that each audio track has a little stereo/mono button. This should be set to mono when recording/playing a single channel/mono source and set to stereo if the source is stereo.
You can not just convert a mono source to stereo by some kind of magic button. However, adding an effect such as reverb that is stereo will result in a stereo sound. It's a good idea to think with the mindset of those who, back in the day, recorded with analogue mixers and multi-track tape machines - every mixer channel and tape track was mono. You would plug a mic or DIed guitar/bass into a single channel on the mixer and record it to a single track on the tape machine. Only if you were working with a stereo signal e.g. synth or stereo pair of mics, would you plugin to two inputs and record to two tracks. On playback/mix, you would pan the mono signals to where you wanted them in the stereo field and hard pan (left and right) the stereo paired tracks. The only difference with Sonar is that you can use a single stereo track for stereo sources instead of two mono tracks.
One simple trick to fatten and stereoize a mono sound, is to copy the mono audio clip to another mono track and nudge/slide it very slightly to the right (10 - 100 milliseconds) and hard pan the 1st track left and the second track right. This will create a short stereo delay effect - the longer the delay, the wider the stereo spread will be, but the more noticeable the delay effect will be, so don't over do it. This works well for guitar for example.