Hi Arlouper,
Short answer - you can't directly.
The subfolders in the CDs section are 'Clones' of the floorplan Viewpoints. A cloned folder replicates exactly what is in the Viewpoint folder at all times, so if you want to add or remove further floorplans into this folder you have to delete or create the original storey Viewpoints.
What you can do is to create another subfolder and place the cloned folder in it. You can now also place any other one-off views that should be in the same catagory in this folder too and keep it all organised.
The views in the cloned folder all generally share the same properties (layer combinations, pensets, scales etc) so when a particular plan is not needed, it can be renamed (rather than deleted) and the set will still stay as a clone. You can see this in your A2 Floorplan set, where the first plan in the list has been renamed.
Hope that helps!