I’m not quite sure what you mean by that, but I thought I’d share that users can send personal messages to groups even if they aren’t in a Personal Message Enabled Groups group.
You can configure who is able to message a group in the group’s settings
But then it’s still difficult for users to notice that they can do this. I added a link to the inbox and links to prefilled messages to some groups into a public custom sidebar section. (I hide it from logged in users with Navigation (sidebar) menu display control)
Before that I had a button in the header or top navigation
Since Personal Message Enabled Groups only prevents users from creating new PMs, your admin might also want to close all existing conversations between users. Maybe the Discourse team can help you with that with some kind of bulk operation.