The master slide organizer shows every slide master in the slideshow. If you've created a new Keynote slideshow, it displays every slide master in your chosen theme. When you apply another theme to one or more slides in a slideshow, Keynote adds all the master slides from the second theme to the master slide organizer.
To add a single master slide to the slideshow, do one of the following things:
In another Keynote document, find a slide that's based on the master slide you wish to import, and drag it from that slideshow's Navigator into the current slideshow's Navigator. Keynote adds that slide to the slideshow and also adds its master to the master slide organizer.
In another Keynote document, find a master slide you wish to import and drag it from that slideshow's master slide organizer into the current slideshow's master slide organizer.