Well, at the very least, there is always ye olde "copy and paste into a new forum topic." I've never run a forum before, so I don't know if there's a more efficient, cleaner way of doing it (sorry).
Of course, not all of the topics have to be brought here, it's really just those important bits of information, like the snippets from the Siren Maniacs books that explain such things as where the 1976 Miyako was in the first game, et cetera, that shouldn't be lost.
This is, of course, assuming you intend to close the Horror Zone forums before too long.