Given all that is involved in the DreamHost Anti-Spam policy, I strongly suggest that you are far better off establishing your announcement list, and simply posting a link to a "sign up" page where users who are interested can subscribe.
This will put them through the approved "confirmed opt-in" process and help you stay clear of spam-related TOS ("Terms of Service") violations.
That said, and if you clearly understand what is involved TOS wise with adding name "en masse" to your list, you may be able to locate a phpBB plugin to "dump" your names/email addresses from the database into a format you could then cut'n'paste into the announcement list subscribers list. You could also probably do this with a phpMyAdmin dump, and a little processing/programming.
If you do decide to go that route, please take the time to read, and understand, the Anti-Spam policy things that are related to adding "manually" to a DreamHost announcement list.
You have a very nice site, and you certainly don't want it to get "hosed" over a few spam complaints.