Generally you build a website around the forum package you are planning to use (such as SMF) unless you are writing the forum software yourself, in which case it's the other way round.
It's not an easy or clean task to patch together 2 different login systems. I think you're going to find it ends up a bit messy. Both login systems will need to have the same encryption on passwords, be able to use the same database setup, etc.
IMO I would re-write the website in PHP, thats the server-side language of the future.
How much does the website do? I might be able to recommend something, or do some PHP coding?