I’ve done it once—surprisingly easy. You can start a new topic from the bbs home page.
Or—in the words of @orenwolf from his meta post Change to inactive topic expiry—
…in the event there was some old topic you wanted to continue, simply start a new topic from the last comment of the closed thread.
My own advice, from experience: Taking care to narrow the topic, and being clear as you can about what you’re inviting people to discuss in the thread, leads to better discussion 
