I see! A portal page, in other words...
Flarum doesn't have that capability at the moment, but the possibility of adding an extension to support that sort of thing has been discussed in the past. It's quite likely that it will be added eventually, if not as part of the default package, then as a third-party extension.
In the meantime, you might consider putting your own portal page on site.com and install Flarum in a subdirectory. Then you could use a header link in Flarum to link to the portal. Check out the "Home" link at the top of this page for an idea of how that would work.