for additem.
You must create in backpack(BP) a function AddItem and that other functions you want
when you hit with a line trace on Itempickable from your character BP , you only need call AddItem using that backpack var in character BP, check before if isValid if not valid then you perhaps not have backpack and you dont SET the backpack var.
Hit->if its itempickable_C then backpack->Additem (Hit Actor)
and then do the work for adding the item to and the Backpack Inventory in the additem fuction
edit: and rember when you drop the backpack set the backpack variable in character to NONE