Some specific searching for this reveals that a couple of volunteers have set up ground-based Flarm receivers in gliding-intensive areas, and convert and upload these signals to FR24. As Flarm range is only 2km or so, don't expect global coverage soon...
It's rather more than that.
The Flarm tracking system is an entirely independent system from FR24 called the Open Glider Network.
There are >50 receivers in the UK (and many more around Europe - all put in within the last year), with ranges sometimes in excess of 50km. See
here.
To get a feel for the coverage look
here
They show up on FR24 because there's a feed between the two systems.
Hardware that can feed FR24 (though not necessarily the FR24 supplied hardware) can also feed OGN, though probably not at the same time.
Paul