The P99 RAD was unveiled at MSPO 2008. As far as I can tell it is a standard P99 with a Picatinny rails and a different, but still interchangeable, grip.
The “RAD” part of the name refers to the hometown of the Lucznik Arms Factory, Radom. Interestingly enough “Rad” was the the unofficial nickname of the Polish Vis semi-automatic pistol used in WWII.
The pistol was designed by the Polish military and I do not know if Walther will sell this design to the public.
Photos taken by Remigiusz Wilk (REMOV).
More here (this page is English, most of site is not)
I have done my best to find accurate information about this firearm on the Polish MSPO 2008 website. I do not speak or read Polish so I had to rely on Google Translate. If I got something wrong, please correct me in the comments.