You can use the code you mentioned from the blog, but you have to change it a bit:

public class SiteMetadataExtender : IMetadataExtender 
{ 
    public void ModifyMetadata(ExtendedMetadata metadata, IEnumerable<Attribute> attributes) 
    { 
        foreach (ExtendedMetadata property in metadata.Properties) 
        { 
            if (property.PropertyName == "iversionable_stoppublish") 
            { 
                property.GroupName = "EPiServerCMS_SettingsPanel";
                property.ShowForEdit = true;
                property.IsRequired = false;
                property.Order = 1; 
            } 
        } 
    } 
}

By using the interfacename where the property is defined in (in this case iversionable), followed by an underscore and the propertyname (stoppublish) you can move more properties to the settingspanel.

Did you succeed in moving the Expiration date to the settings pane? The shown example uses a different property. The correct name of the property is the trick and i couldn't find it..

The above code should still work

Did you also register the metadata extender in an InitializableModule?