Hello petertough,
Thank you so much for using our theme SuperMag.
There is no any options for selecting more than on category on slider. You can add the more post on the same slider.
If you are familiar with WordPress code, you can customize theme with Child Theme approach. We have lots of options for slider on our premium version of theme SuperMagPro. You can use slider through category, page or make your own custom slides and also have the options to increase the number of slides too. http://www.acmethemes.com/themes/supermagpro/
Best Regards,
Acme Supports