how to share other DB users wirh WPMU

Hellow,

I think there is away to connect other database users and their password only with WPMU,

I mean every user at that DB will be auser in WPMU but he dont has ablog, he can create it later ...

but there is WPMU DB already exist so to idea to connect with other DB only,

can it be done?
I know about programming but Iam not proff. so I need help to do that ...

any help??

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