already exists, here's one example: https://apps.repebble.com/en_US/application/56205296ece6340e...

or a search for 'voice': https://apps.repebble.com/en_US/search/watchapps/1?query=Voi...

it's just a button and a mic, which both the watch have, so seems like it would be trivial to implement even if they don't do it officially