When Right-Clicking on a Legend in the TOC there is an option to remove from Legend. Please Add the Option to "Add Item to Legend".
Dragging items from the TOC up into the Legend TOC is very clunky and since there is already a "Remove from Legend"just include an Add Item To Legend with a drop down list of the layers available in the map.
Dragging ANYTHING for any reason in the TOC is very slow and not at all smooth. If things dragged smoothly and quickly I'd appreciate the functionality but it's almost unusable as it is with maps that have a lot of layers in them.