If this were a recording of a real kit, then using SONAR's convert-to-mono feature would be the proper approach. However, virtual drums are a special case. Here, it's best to take care of the mono-izing within Superior Drummer's mixer. Anything you can get right at the source is better than fixing it later.
(I'm assuming you don't want
everything mono. More often, it's just kick and snare most people want in mono, retaining the ability to pan toms and overheads.)
For the kick and snare audio tracks, choose mono outputs. Here's what I usually do...route all three kick mikes to Output 1/2 in SD's mixer. Then for the input to the kick track, select "S1/2: Left (mono)". I then do the same for the snare: all the snare channels are routed to Output 3/4, then select "S3/4: Left (mono)" as the source for the snare audio track. Make sure those two audio tracks' interleaves are set to Mono.
(Another step I usually take is to alter the panning of the toms, as their default pans are too wide for my taste. A real drum kit isn't as wide as the whole stage!)