Actually I have already written a complete set of Delphi components for TTS and ASR (SAPI 4 and 5), and after several years I didn't decide to distribute them yet.
Anyway I don't see any clue to add TTS support to Office Assistant package. As you could find there are some free TTS components that you can use with the assistant. You just need to define a function that calls Assistant.Speek and TTS.Speak together.