In a single map, you can't.
In a campaign, you can import a new mdx file, or new blp files with the same path as the original files, this way they will overwrite the original loadbar. Files for the loadbar are located in UI\Glues\Loading\Loadbar.
That doesn't work with a single map because the imported files get loaded with the map, which means they are available only after the loading of the map.
Whereas with a campaign, files get loaded when you open the campaign, so they are available when you load a map.