As far as I know, there is no way to do this by default. You will have to create a script to do this.
After you create the ammo box, remove all the items.
Then you can randomly select items to add. For example
_wep = weaponArray select(floor(random count weaponArray));
_veh addWeaponCargo [_wep, 5];
Where weaponArray is an array that contains the classnames for the weapons from which you want to select.
This would add 5 of that weapon into the ammo box.