Possible to subscribe to all topics in a forum invision comm

[expeditedshipper]

We have a user set up to receive emails for subscribed forums. It will send a notification to that email which will then send a message to a Slack channel which is attached to that email. 

It's working fine if there's a new post in one of those forums, however, when someone comments on one of those posts, it doesn't notify the slack channel. I have learned this is because that user would need to be auto-subscribed to each of the new postings in each forum.

Is there a plugin that will subscribe a user to each new posting in a forum? Also, can we retroactively subscribe that user to ALL postings in each of the forums? Thank you.