Hi Yakuzaa,
I believe there is only way to accomplish this: typing manually after <#>:
Page <#>/TotalSlides
TotalSlides is the number of slides that you already know. So after final review of your presentation, you can change this field in Slide Master and make it show the Page x / y.
I know I am probably repeating what you already know but this is the only way that I know.
I hope it helps.
Suat