I'm not sure about the animations, but you could just have your "trolley" have several tote objects in it at the beginning, and load items not directly into the task executer or trolley itself but into the totes inside the trolley based on whatever loading criteria you have. You could edit the tote flow item's look as well if you wanted it to look more like a rack inside of a trolley. Would something like this work?
Alternatively you can look inside the pallet object in your flow item bin, and click on the packing method to get the code on how FlexSim stacks boxes on pallets. You could create a similar "packing method" on a pallet in your trolley, but edit it so that the z location of items would be based on whatever criteria you had. I'm not sure if this would work for your situation either though.
