Do the CHE editors seek out ideas for articles from these fora?
Sometimes.
And if the editors get an idea for an article from the fora, do they invite posters to contribute
Sometimes.
or do they have a stable of authors to draw on?
Yes.
Crossover from the fora to the rest of the CHE happens in a variety of ways: by solicited contribution ("I'm a writer for the Chronicle and I'm writing a story/column about X; you are invited to share your thoughts about X for possible publication"), by someone from the CHE staff noticing an interesting idea for a story and contacting forumites (via PM or email) whose experiences are relevant to that idea, and by other kinds of cherry-picking as well. And probably in other ways I don't know about.
VP