It is going to be tricky to get the page content that is created by the Custom Facebook Feed plugin to be formatted correctly on Discourse. I would like to come up with a general solution to the problem that allowed complex WordPress markup to be pulled into Discourse when the “Show Full Post” button is clicked on Discourse. If this was possible, the solution would be to publish an excerpt to Discourse, instead of publishing the full post. Users would click the “Show Full Post” button to see the full post. The main issue I am finding with this is figuring out a way to allow iframes to be pulled to Discourse with the expanded post. Youtube videos are displayed in an iframe on WordPress. Videos are currently being removed from the HTML when the “Show Full Post” button is clicked.
With the current Discourse functionality, I am not sure what the best approach for you would be. One possibility would be to publish your Facebook group posts to Discourse via Zapier. There is a guide for setting this up here: Using the Zapier Discourse Zap Templates. The Zapier template for publishing Facebook Group posts to Discourse is found here: https://zapier.com/apps/discourse/integrations/facebook-groups.