You can just import the posts without the users. The posts would then be linked to profile number 0 (which is a guest account.) You'd still have all the usernames attached to the posts though because that's saved seperately in every post.
Anyway, if the devs ask us for it, we'll certainly give them a backup. The LFS forum at RSC won't be going anywhere though, so you can still visit it and read the old posts.