Where's The Board?
Is there a BGG entry for the expansion? I'd like to subscribe to it so I know when it's coming out.
In the 'Information' section of any game's page, the first section detailing the game, there is a sub section called 'Family'.
If you click the game's name in that section it takes you to a new page showing linked items. Subscribing to that page will subscribe you to all games in its family.
I'm sure that's how it works. So if you subbed to Catan's family page you would be also subscribed to every separate Catan game, expansion etc.