I think you can, actually. You need to have TTS Service Extended installed from Google installed from the marketplace. Then open it in the App Drawer, change the "default engine", and check "Override App Settings". The only thing you have to do is to add new engines ... which I don't know how to do or if you can do at the moment (but it seems like you should be able to).
I'm just guessing, though. I could be totally off-base.