How to allow insertion only at a certain position?

Hi there!
I'm sorry for asking questions but I'm not sure of how to do this. I have two lists: A and B. None of them is vertically Sortable, but I want to be able to drag elements from B to A (clone them, in fact). However, I don't want to allow the new-coming items from B to be inserted at any position in A, I want them to be added only at the bottom of the list. Is this possible? 