Using the openfire.xml config file (which lives in the \openfire\conf directory), it is easy to allow admin console access to specific Openfire users.
Under the tags, respectively. The contents of these tags should be a comma delimited list of usernames (or JIDs).
Your openfire.xml config file might end up looking something like this:
Note: users defined in the list (users that have accounts on other servers but can still access the server in question).