The only way I can think of to ease your issue, is within 7.3 is to do a member search, with the optional extras such as 'is not banned', 'post count over xx', optional field info, last post date etc., to form a list of members, then tick all those that belong to this passworded forum, and then add these people to the new group (make the new group first & set up permissions for that group)
What you could also do to make things even easier is to set up one of the optional fields to say 'Password group' & make a post in the password forum asking all those members who can see that post to populate that field with a code word -then do a search for these people as above, cross reference with your list and then add them to the new group.
Just a couple of thoughts.
