If you have items in the gradebook that are not created using the gradebook (i.e. created from the Assignments tool – when you display the item edit page for the item, the source is not 'gradebook'), when you right click, in the gradebook structure pane, on the name of the non-gradebook sourced item, then the Add Item panel that comes up will not allow you to enter a name or other identifying data (the add pane is in read-only mode)