I don't know anything about TTS in Cp, but I'm interested and will look into this myself. Before you get no response at all, here's a thread dealing with the issue. It's three years old, but can be a starting point.
Sorry but that old thread is not at all linked with the question by the OP.
Any installed voice on your system can be used in Captivate. If you are on Windows the Win voices automatically appear in the TTS window, even before you install the TTS voices from NeoSpeech packaged with Captivate.