What I found out the other day and I am not sure if you are looking for a similar solution but the forum has an API which if enables can be used to login the user from your existing site.
For example, in your current login script if you add the following:
session("user") = user
session("password") = password
Then view the forum, it will create the user in the forums database if it does not exist or it will log the user in if it exists.
This is very easy to implement, let me know if I have miss-understood you.