Shared database, only users, possible?

[Eagle Creek]

Howdy!

I have a question. I'll try to explain it so clear as possible.

I'm going to set-up 5 new vBulletin forums. So far no problem. But here it is.
My client wants to have a single-registration for all forums (if possible with selection).
So for example. I have forums A-B-C-D-E.
A user is browsing the web and sees forum A. He clicks "register" fills in the forum and then: he can tick checkboxes on wich forum he wants to be registered also. So he sees on forum A:
"On what forums do you want to register
A
B
C
D
E
"

Something like this.
The user is ticking the boxes A C and D and he is registered on that forums.
OTHER OPTION (maybe easyer) IS:
just register at all forums at the same time (no checkboxes at all). So, somebody sees forum C, starts to register and when he access forum E he just can log in and post..

BUT:, and this is the hard part I'm affraid..
The other foruminformation has to be seperated!
So when I give "Thomas" A nickban on forum A, he just have to have access on forum B-C-D-E. And when I make "Thomas" moderator on forum D, he still have to be a normal user on forum A-B-C-E.

Is this possble to do??? I heared it was a lot easyer in 3.5 then in 3.0.7 but how?
I know don't much about MYSQL so if possible, explain it easy to me.