If this is a soft synth then change the track to mono. If you're using a Simple Instrument Track (i.e. a single synth track which combines audio and MIDI) then you'll have to split the track into an audio and MIDI pair before you can make the switch. It's kind of crazy that Sonar removes the stereo interleave button on Simple Instrument Tracks, but what can you do. Just right click on the track, select "Split Instrument Track," then on the audio half of the track in the console (or in the inspector), there should be a stereo interleave button right above the mute button. It will show a two circle icon for stereo, and a one circle icon for mono.
If you then want to recombine the audio/MIDI pair into a Simple Instrument Track again, just right click on one of the tracks and select "Make Instrument Track." It's worth remembering that to combine an audio & MIDI track into a Simple Instrument Track, both tracks must point to the same synth. So if you change the routing of either track after splitting them, you won't be able to recombine them unless you change the routing back.
However there are other options too, in addition to the ones Craig mentioned above. There are plugins which can center bass frequencies below a certain threshold on a stereo track. TP Basslane is a free one that works very well and you can download it here:
http://www.toneprojects.com/products/plug-ins/basslane/ A plugin like this would be ideal if, for example, you have a cool sounding bass patch that has some interesting stereo information in the higher frequencies. You could set the threshold on TP Basslane to something like 150Hz, then everything below that frequency (i.e. the bass part) would be perfectly centered while the higher frequency stuff would retain its stereo placement. You can really give your tracks a lot more "oomph" with this plugin, I love it.