XF 1.1 Easy way to connect existing user accounts to forum accounts

Hi all!

I would like to ask, if there is any easy method to connect existing users from our portal to forum user account? I mean is there any tool to integrate such process?

Currently we have 6000 users in our Portal and would like each customer to have a button: (REGISTER ON FORUM) which would create a user account on the forum or connect to an existing forum account.

In the same way on XenForo works the facebook account connection or the customer and community account connector.

Is there any ready solution for that? I don't want to reinvent the wheel but I can't find anything via community search.