We have 2 options to achieve this.
You can use the BB forums in LearnDash lessons, and then the comments show up in the Activity Feed. To insert a discussion, use the shortcode [bbp-single-topic id=28951] and replace the ID as needed.
You can use the WPDiscuz plugin.
Note: If the plugin is not listed on our integrations page: https://www.buddyboss.com/integrations/, we cannot guarantee that a third-party plugin will work well with our BuddyBoss Theme as we haven’t tried it extensively and also we don’t have an integration nor a built-in function with such a feature yet. You are free to try, and we are happy to know how it works on your end.