I could be missing some non obvious way of accomplishing this.
But, it seems that there is no way to add a magazine on a vehicle turret with a specific amount of ammunition.
There is:
https://community.bistudio.com/wiki/addMagazineTurret
e.g.
_tank addMagazineTurret ["SmokeLauncherMag",[0,0]];
But that does not take ammo count.
Ideally there should be a command similar to:
https://community.bistudio.com/wiki/addMagazineAmmoCargo
but that would work with vehicle turrets.
Syntax:
_vehicle addMagazineAmmoTurret [_magazine, _count, _ammo, _turret];
Sample usage:
_tank addMagazineAmmoTurret ["SmokeLauncherMag", 2, 5, [0,0]];