I think they are already there, just a little hidden. Drop into the sub-forum you are interested in the feed on, and look at the available feeds again.
Eg:
If I look at the available feeds on the main page I get:
Click to subscribe...
Feed - St. Louis Brews
Feed - News
Feed - All forums
Feed - New Topics
Feed - Active Topics
If I drop into "Beer, Cider & Mead Discussion" I get the same with another entry at the bottom:
Click to subscribe...
Feed - St. Louis Brews
Feed - News
Feed - All forums
Feed - New Topics
Feed - Active Topics
Feed - Forum - Beer, Cider & Mead Discussion
So you need to go into each subforum you want to add to your reader.
Took me a while to figure it out it was already there.

j