When you open your Sonar project, it will warn you about the missing VSC. Just ok that message and proceed to load the project. Once you're in, open up the DXi rack and remove/delete VSC, load TTS-1. It will create 2 additional tracks by default, a synth output ttrack and a midi track assigned to the TTS-1 which you can disregard.
In your track view, expand the tracks until you see the midi output dropdown. Simply assign every midi track to the TTS-1 and you're done! If you've used up all 16 channels inn TTS-1 and you need more, just add another to the DXi rack and sequence to your heart's content, or do what I do and use a different synth altogether (some of my projects have 4 or 5 synths, a mix of hardware and software.).