They did make a P99 in 9x21, however it is very rare in the U.S. You'd have to special order it. Probably from Earl's. There's also a .357 conversion barrel. Do a search for it and you'll find the topics on it.
I'd like a 357 SIG pistol, but not a drop in conversion barrel. I wasn't certain about the 9x21mm though. It seems that Walther can be on the cutting edge and the most conservative firearms company in the world.