That really depends on whether the third party has implemented their voices in a way that's compatible with Microsoft's implementation and, hence, can be retrieved in the same way. Even then, the voices available are language-specific, and even the Windows defaults are limited - although my PC is fully-configured for Australia, the only options I get via:
Control Panel>Ease of Access>Ease of Access Center>Use the computer without a display>Text to speech
are Microsoft David Desktop and Microsoft Zira Desktop, both of which have US accents, even though the narrator itself lists numerous other options. The Microsoft James Mobile option, which supposedly has an Australian accent, sounds like no native-born Australian I've ever heard - it's actually more like a British accent.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]