Sitemaps were not designed for the purpose of indexing every single possible page on a site. Their purpose, however, is for providing quick access to content which is "hidden" or obscurely placed. phpBB, and forums in general, do not fall into that category because the board, categories, forums, topics, and posts are structured already. Many search engines specifically cater their spiders to be able to crawl topics more effectively. For example, many search engines know when the last post in a topic was made and display that on the index. It wouldn't know that information if it didn't browse to the last page of the topic.
That being said, in order to accomplish this, you would need a MODification. There is already an existing Sitemap MOD
, though I'm not certain if it goes to the extent of indexing individual posts. You'd want to check with the MOD author in that topic.