For adding a weapon to a player's backpack, you can do:

(unitBackpack player) addWeaponCargoGlobal ["hgun_P07_F", 1];

However ... there are no equivalent commands like:

(unitVest player) addWeaponCargoGlobal ["hgun_P07_F", 1];
(unitUniform player) addWeaponCargoGlobal ["hgun_P07_F", 1];

Basically this is a request for developers to add two new functions:

unitVest, and unitUniform that behave just like unitBackpack does.