@Shaun Thanks but unfortunately the RSS feed plugin seems to be having some issues (at least for me), particularly with not grabbing all posts and/or duplicating them.
I'll keep my eye on the rss thread and see if it gets any updates.
I understand what you're asking for. The reason you aren't getting an answer is most likely because no, they're isn't a ready-built plugin with exactly what you want.
There area couple close ones, including the iframely plugin, but none with the popup feature.
@juan-g If I’m not wrong this is only for groups. Anybody can't make subforum both with his own group.
This is interesting idea for plugin or like a feature for new version of core nodebb.
Good for wide community forums on autopilot, when owner not have enough time to support all desires of members.