That's an interesting question, indeed. I don't know if I offer the best solution here, but a clear path is as follows:
Generate your CBC plan for just the 8 competitor's brands. Then, export that design (from the Design tab) to an Excel-compatible file (.csv). Then, using random functions within Excel, randomly pick one of the concepts within each CBC task to set to your client's brand (brand #9).
Now, use the File + Save As function within SSI Web to create a copy of your SSI Web study. Modify that study to add a 9th brand to the brand list (your client's brand).
Reimport the modified design file (again from the Design tab) into your new study, and re-test using CBC's test design functions.