MyBB Community Forums

I have a site, which you register on for a service, and I also want it to have a forum.  I would like the users to not have to regisiter on the MyBB too, like I want accounts created on the register page for both the site and /forums

Thing is, the original site uses SHA1, while MyBB uses something different.  How can I accomplish this and also move all current members to the MyBB forum too?

Moved to Third Party Integration.

To help you further, MyBB uses md5((md5($salt).$password).

If your passwords are sha1 hashed, best practice would be to save them into MyBB database if the user does login the next time because you'll have the plain password then.

The other option would be to force all users to reset/change/update the password

You aren't allowed to store user passwords in plain text. What would work though is hooking into the login function and storing the sha1 of the password that is input after validating the password is correct.