The game really only can handle the "book" values for things. it is a fact that is verifiable. Different ammo loads are allowed e.g. by having the CS tank variant of an MBT, with a more anti-grunt loadout, more prevalent in MBT though (e.g. a T64 with more HE, less AP and no tube ATGM).
In real life, crews did load extra ammo. The tanks at Beda Fomm had well over the book value - the extra 2 pounder rounds were on the turret floor. US Sherman crews sometimes loaded extra loose ammo, and it was this that helped the "tommy cooker" label for the vehicle, as the unprotected propellant charges would contribute to a "cook off" on penetration.
The best way, is to use the scenario editor, should you have info that the tanks at X real-life situation had an overload of ammo, and edit the ammo load. Build a scenario where the special situation applied, in other words. And possibly, you might be able to reduce the survivability number as well, to reflect that having loose cartridges sloshing about the crew compartment was not healthy!
Cheers
Andy