There is a documentary on youtube which says about 108 stugs were assigned at the start of operation Barbarossa. (search for 'german war files stug iii'. There are also for several others in this series). It says they were involved in heavy fighting from the start, and were successful as both infantry support and as anti-tank even vs t-34's.
Anyway, it should be easy to patch into the game if the developers want to. I saw the rules for 'price of honor', and that only has the stug iiig, not the earlier versions.
The counter list in Price of Honor includes the counters from Awakening of the Bear and Storms of Steel, so the Stugs are actually from Storms of Steel. But making other units, like Stug IIIb, is probably very easy, if someone makes up stats for them, and that someone should probably be the designer of the board game, Uwe Eickert. I know there is some research behind the stats, and new units should be researched in the same way, if they're to be an official part of the game. This research will probably only be done if he were to make scenarios with those units. That's why I think this is best left to the modding community, as mods are unofficial.