This plugin lets you assign non-authors as post author.

Martin Teley

Do you have a blog/website with a lot of contributing authors but you do not want to give them the rights to publish 
the articles themselves for whatever reason? In that case it would be nice when you are able to give them credit for 
what they wrote anyway. Since WordPress cannot override the author with a non-author by default, this plugin overrides 
the Author meta-box with a meta box that can select any user.
So whenever you want to place an article that was written by someone else, you just create a user with subscriber role 
(if he/she hasn’t already) and assign the article to that user!

Change Author

This plugin attempts to override the default author meta box, but that doesn't seem to work any more. However, it is possible to simply filter the options for the author dropdown list like this to get the same result:

function add_subscribers_to_dropdown( $query_args, $r ) {
    $query_args['who'] = '';
    return $query_args;
}
add_filter( 'wp_dropdown_users_args', 'add_subscribers_to_dropdown', 10, 2 );



More details in the user contributed notes for the wp_dropdown_users_args filter.

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