This is What I got so far, but it is only one backpack with gear you would need a new file and new add action for each backpack if you do it my way...
//PV2 Derienzo [2nd ABCT] Items in Backpack in Cargo Script
player addbackpack "b_kitbag_base";
(unitBackpack player) addItemCargoGlobal ["",1];
(unitBackpack player) addMagazineCargo ["",1];
What that does is it adds the backpack to the player, and then adds the items to the backpack
this addAction["<t color='#ff1111'>Backpacks</t>", "backpack.sqf"];
Like I said before, you need a new addAction script for each backpack if you do it this way.
If you have any questions, let me know!