My apologies I think I got the name wrong. I was referring to the "Number Area" box which can be added to slide masters.
Adding a slide number rectangle won't be any use if the end user add/removes or otherwise manipluates the slides. Using the "Number Area" box would mean the slide number updates automatically within PowerPoint.
So my question was can you manipulate this object using Aspose.Slides. When directly editing a master within PowerPoint you can add text to display with the slide number, for example "Slide number <#>" and the <#> is converted to the actual number.
Please let me know if I am still being unclear.