Quixote,
Sounds like there may be two options... If you can get the Elk to trigger the TTS you're using already that'd be ideal. What's doing your TTS today? If there's some form of controller that can monitor the elk, there may be options. The Elk can send ascii strings, and lots of home controller software can do more advanced monitoring/triggers.
If not - depending on just how many voice prompts you're going for, there's another option. Personally I wanted to use some custom sounds beyond what the Elk puts out - so I tied an Elk 124 recordable voice module inline with my Elk keypad speakers... and I hooked them into the 8 onboard voltage outputs on the M1. Using the computer to program the sounds, I'm able to get pretty clean recordings of anything - music, sound effects, TTS played back, etc. However, that option would be limited to 8 custom outputs for things you use often.
-Todd