There are two parts to this.
1. Scraping the forum is extremely easy. Start with the member list, then section, thread, posts. Store as csv or in sql. Very easy to pipe data into new forum db while maintaining referential integrity.
2. Keeping forum updated/synced. Slightly more difficult, but basically the same scraping script, cron jobbed every 5 minutes on the forum's 'new posts' search results.
There is almost no logic involved here, only when posts by a new user are discovered is there any branching involved.
Also set user agent of scraper to googlebot to stay under the radar.
edit: as far as being ethical/legal etc, I don't see any difference between creating an autoforum and creating an autoblog.