The common opnion is that they are not supporting other voices (even the ones from neospeech such as Julie). We only use the TTS option for the development and review stage so once the script is finalized you could export the script and either use an actor and sync the audio or look at a text to speech pay per script service (Neospeech do appear to provide this but at what cost???)
One of my colleagues has had some success with this but it is a long drawn out process - if I recall correctly, having bought the voice he exported the captions into a Word document and then recorded the new voice using Soundbooth if I recall and then imported the individual voice snippets back into Captivate. Took a long time.
As Adam says we use the Captivate voices up until final sign off of the script in demo versions before getting a voice over done by a suitable person in the published version.