There currently is no way to load a specific weapon by script.
addPrimaryWeaponItem, addSecondaryWeaponItem, and addHandgunItem only read from the cfgWeapons class making it impossible to add a magazine directly into the weapon's magazine slot.
It hinders features for both addon-makers and mission-makers with that lack of fine control over weapon-items.
Please either expand the above commands to also read from cfgMagazines, or add new commands to specifically add magazines into a weapon. Preferably these add-commands would be accompanied by commands that let us see what magazine currently is loaded (other than currentMagazine which doesn't work for weapons not in hand), and that let us specifically remove magazines from weapons.