i have tried it also but it isn't an out of the box option, so I used a workaround:

run this SQL script and adjust the values:

DECLARE @Username nvarchar(256), @UserID as int, @aspnetUserID uniqueidentifier, @PortalID as int, @ApplicationID uniqueidentifier

 

--enter the userid here

SET @UserID = 4

--enter the portalid here

SET @PortalID = 0

 

SELECT @ApplicationID = ApplicationID

FROM aspnet_Applications

WHERE ApplicationName = '-1'

 

SELECT @Username = Username

FROM Users

WHERE UserID = @UserID

 
 Ok, I wrote the SQL to convert a user to a SuperUser.  You need to run this in SQL Query Analyzer...there are two parameters you need to specify (UserID and PortalID).  Backup your database before trying this, and try it in a dev environment first.  I have tried this locally and it does the trick.
---------------


SELECT @aspnetUserID = U.UserID

FROM aspnet_Users U

INNER JOIN aspnet_Applications A

ON U.ApplicationId = A.ApplicationId

WHERE U.Username = @Username

AND A.ApplicationName = convert(nvarchar(256), @PortalID)

 

UPDATE aspnet_Membership

SET ApplicationID = @ApplicationID

WHERE UserId = @aspnetUserID

 

 

UPDATE aspnet_Users

SET ApplicationID = @ApplicationID

WHERE UserId = @aspnetUserID

 

UPDATE Users

SET IsSuperUser = 1

WHERE UserID = @UserID

WOW! Thanks - just tried it to my 3.2 install. Worked first time.
Now all I have to do is figure out how I can enable AD users to logout ... 

Chris