You would probably just need to access the JForum database during the authentication phase and check against the appropriate tables that the username and password match.
Given that the passwords are probably encrypted in the DB, you would need to know the mechanism that JForum uses for that and use the same one so you would actually compare the encripted passwords instead of the clear-text version.
Looking at the source
you should be able to find which tables you need to access and the encrypting mechanism, so everything is there.
Good luck
[originally posted on jforum.net by GreenEyed]