G2 & Board (vBull or phpBB or or..) -> Usersharing: Users from G2!

Hiyas...
I've got currently a G2 running with lots of users and pictures. These people ghave themselves registered directly into G2. Now I'm looking for a board as an addition - BUT I want my currently registered users to have board accounts so they don't need to re-register there. -> The board (vBull, phpBB whatever) should use the userdata from G2.

I've already read several postings here about Integration but they all sounded like "you got a board and then put in some G2 features and G2 uses the same user then your board" - that's not what I'm looking for, and I don't need direct "integration" - just user sharing.

Possible? If yes: A link would be nice to save me some more h's of searching (maybe at the wrong spots)

Thanks! :D