You could do this with a tablet or phone (if you have a wallmounted tablet, this is perfect for that) via LANnouncer.
You would connect the tablet or phone to Alexa via bluetooth so all audio from it comes out of Alexa.
You would add the tablet or phone via LANnouncer as a speaker. This will also support the siren feature.
http://www.keybounce.com/LANdroidHowTo/LANdroid.html
If you’re using SHM, you could then add this as a Siren in the SHM setup (Security > Alert with Sirens > select your device).
You could then use the Big Talker app to add voice statements if you wanted. [OBSOLETE 1.1.6 5/25/16] Big Talker - Talk when events occur