Problem is that German forces would have stopped using this weapon around 1942 or so.
They also wouldn't have used stun grenades though, and Soviets infantry would have been snorting PPShes, and Americans would have had mortars, etc...matching history in this game's more of a frequent bonus rather than a design decision adhered to.
Actually, Panzerbuchses are already in the game, they are used inside the Theatre of War missions. This would indicate the Wehrmacht and Soviet factions are already all fully-functional with the weapons, but it's possible the USF, UKF and OKW's units weren't made to use them considering in official content they currently can't interact. One could probably test if that is the case or not with the CheatCommands mod, since I think it allows you to drop those weapons and play around with them, but I haven't done so myself.