suggestion:
This addItem ["slot","classname", x]
This addMagazine ["slot","classname", x]
slot - name of the slot or number (inner to out 1 = uniform; 2 = vest; 3 = backpack)

It's good idea too :)

this addbackpack [backpack name] still works like it did in Arrowhead

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having a command like unitBackpack for the vest and uniform would be easier.
Creating a object you can use it with addWeaponCargo, addMagazineCargo and addItemCargo.

(unitVest player) addWeaponCargo ["Whatever", 10];

+1 for Ferit's suggestion

It would be easier, if they implement a loadout editor, that you can drag and drop all available guns, items and backpacks into the unit slots.