The simplest way I can think of would be if all users are member of a specific channel, then you could simple send a @here message on it, but aside from that I know of no built-in method to achieve your needs other than making it your self using the API, you could easily write some code to do that on pretty much any language or even using curl directly.
You would need the following methods:
https://api.mattermost.com/v4/#tag/authentication
https://api.mattermost.com/v4/#tag/users%2Fpaths%2F~1users%2Fget
https://api.mattermost.com/v4/#tag/channels%2Fpaths%2F~1channels~1direct%2Fpost
https://api.mattermost.com/v4/#tag/posts%2Fpaths%2F~1posts%2Fpost
So basically you authenticate, then get a list with all users, then generate a DM with each or a GM and then post a message to that channel using its ID.