Need it .... Or can i change WHO posted a comment to a topic manually via adminsurface? A member posted a comment and forgot to log in - would like to change this ...

Topic question: You don't. Users can do a password retrieval, but webmasters have no access to the passwords, since they are stored in encrypted form.

Wow, what a service Changing the author of a comment just because he forgot to login. You can do that through the use of the PHPMyAdmin module, where you go to <prefix>comments (I think) and find the comment. There you change the uid of the comment to the one of the correct member.