As I should have guessed, it isn't possible to import the user data directly from this forum, because the passwords are encrypted. We'll have to either create a registration page or create users on demand, and for now have to deal with multiple logins.
The best solution, though not easy at all, would be to create some sort of Singe SIgn On for all webapps in opengoo.org, which includes Simple Machines Forum, Doku Wiki, OpenGoo and maybe WordPress (with Subscriber roles).