You may be able to do something with IFTTT. I have an “applet” which turns on various lights when the front door opens – though I am using wireless tags rater than magnetic switches. At the moment my door sensors are armed at 11pm and disarmed at 5am. I haven’t tried it, but I think they could be linked to an echo so that they were armed with a voice command.
Having said that, the echo needs a trigger word to start it working – usually “Alexa”. I don’t see how you could set it up to say something spontaneously.