Unfortunately, no, you cannot. Notification settings are controlled by the users themselves. A workaround you can use is to "tag" the channel in each new post, so that every member is notified (by default triggering a notification).
You should also vote this up over at the Feedback portal: https://feedbackportal.microsoft.com/feedback/idea/f8fc1fbe-dbb3-ee11-92bd-6045bd7b0481