Just depends on your use case.
At the present time (nothing to do SmartThings, this is just how Alexa works) the way you would do this is with the Alexa Skill kit (ASK)!and set up your own server. It’s the same way that Alexa plays magic eight ball or gives you your horoscope.
So it’s doable, but a fair amount of work.
The Issue I see with it is amount of time you’re standing outside your door waiting for the challenge and response and second response. In the same amount of time you could probably just open the door in any of many different ways, including a watch widgit or taking out your phone and just toggling the device. So I’m not really sure I see the practical application of it. The usual goal of voice-based lock systems is to reduce the amount of time it takes to get through the door. Not add to it. So I’m not seeing this as meeting the “convenience goal” for most people, although it might for someone who had no other way of opening the door. But you’re keeping the person on the street for a pretty long time.
If you remove the “convenience goal” and view it as a security checkpoint where making the person wait for the next stage was acceptable, then it could work for some sort of antechamber set up where there was a safe place for the person to wait while the whole procedure completed. Could be cool for a treehouse or kids’ playhouse kind of thing, too.
So if you really wanted to do it, ASK would be the way.
The following project report is from a community member who’s been using ASK with SmartThings and Alexa so you can see how that might work together.