Adding Picked Up Items directly to an Items Inventory

  Hi, I am trying to create a flower bag similar to botania but with generic flowers.  I am using itemPickedUpByEntity trigger and trying to add the item to The inventory of the bag instead of the inventory.  I have tried nbt tags and using set itemstack to slot in item if it has inventory.  However,  when I open the gui I do not see the item.  The NBT tags are reset each time i call the procedure so it appears the reference to the item is boxed.  How do I reference an item in the players inventory from the itempickedup trigger and add an item to it?