Bb is a WHOLE step below C, not a half step.
Sonar's built-in pitch shifter may or may not do what you want. Certainly seems extreme, when you could just play the part a whole step higher. Orchestral players do this all the time when they encounter older classical pieces that call for a C trumpet. As a french horn player, I had to routinely play parts written in F, E, Eb, D, C, and occassionally A or Bb. But even if you can't do it at sight, just grab a piece of manuscript paper, and write it out.
On an interesting side-note, I transposed a french horn part up a full octave using Sonar's pitch-shifter just as an experiment. It no longer sounded like a horn, but actually sounded very much like a flugal horn/cornet hybrid, and I ended up using it in one of my songs.