merge user accounts zencart/xmbforums/phpbb

I have a Zen Cart shop where products are places on events; an xmbforums forum which has forums & also has an [login to view URL] page which displays event attendees, availability etc from the zen cart shop & also allows the admin to create events(products) in zen cart from the xmb forum; and a phpbb3 forum which is currently totally separate to the other 2. Put simply, we would like to use the phpbb3 forum instead of the xmb forum for the forums, however, we would like to keep the [login to view URL] page feature from the xmb forum as this also updates to external websites & works well with the zen cart shop already - for example when a customer makes a purchase in zen cart they are added to the event in the [login to view URL] page of the xmb forum, along with their payment date - So we want to link to [login to view URL] from the phpbb forum to access this. Currently user accounts in zen cart use an email address to login, and xmb uses a username - they both use the same password, but do require logging in separately. If a customer wants to change their password/email address they do so in zen cart & the new password works in xmb. To use the 3 websites together we need: current users from our zencart/xmbforums installs added to the phpbb site(there is only 1 set of users as aforementioned), preferably to a specific member group so we can keep members who register with phpbb instead of zencart separate; have the usernames/full names/email addresses/passwords of these members synchronize between the 3 websites; have new users in zen cart additionally created/added to phpbb to the specific member group, whilst they also add to/work with xmb as they do currently; have users automatically logged in to the xmb forum(or all 3 sites if the solution is simpler) if logged in to the phpbb forum so that they can see & use the events page without having to log in twice, as if the events page was part of the phpbb forum to begin with. All 3 sites are PHP MySQL based.