When using Library Reading Lists, you will see an area called "My Collection." This area is a sort of "holding tank" where you can add citations for review or until you are ready to add them to a particular list. You add readings the same way as those found on the Add Readings/Citations to a List. Instead of choosing a section on a list, choose "My Collection."
To move something from My Collection to a reading list, click on the ... icon on the right side of the citation. Then click on "Suggest this item." You can then select the reading list to which you want to move the citation.