yes, it's possible. you make your own registration page but you use your forums database for storing the data.
try the tutorials forum. You might want to look up there a bit.
its easiest in a php portal...everything is connected and all the info is pulled from another part of the site (usually forum)
i just did my login script from the phpbb plus...hehe...
What forum system are you using? phpBB? SMF? myBB? Something else?
If you tell us what you're using, we'll be able to help you.
Basically, the 'Registration' link on your site will link to the forum registration page. When someone logs in, it gets the data from the users table in the forum's database.