Thanks.
I see, now, how the whole "Add Topic" function works; if your thread isn't posted in a forum that contains a specific topic, you cannot add that topic to the thread. Being in General/Chat, the topics available are listed here:
http://www.freerepublic.com/focus/f-chat/topics
But that list isn't the same as the topic list available for News/Activism threads:
http://www.freerepublic.com/focus/f-news/topics
Look at both pages and compare the items under "General Interest". "Vanity" is listed as a News/Activism topic, but not a "General/Chat" topic, so it cannot be added to this thread. OTOH, I was able to add "Miscellaneous" because it appears in both lists.
Maybe what's most confusing about this is that the "Topics" link on the FR homepage doesn't navigate to a "master list" of topics, but links to the "News/Activism" topics list, instead, which are only applicable to threads poted in "News/Activism".
BKM (sorry for the corporatespeak) is to navigate to the "Forums" page, and pick the "topics" link displayed to the right of the forum in which the thread is posted to which you want topics added. Then, all of the topics on the displayed list will be valid for the thread you're targeting.