Add Colors to Color Library
You can include as many colors from other libraries as you like, selecting and dragging them to the new space. When the color is in the new space, this will become highlighted, and then you must click the key ⌘ (macOS) or ctrl (PC) so that it stays.
Colors from opened designs can also become part of the new library. Just select them with the color picker, and they will show on the bottom right part of the window. Then drag it as explained before. One last option to include colors in a library is selecting them from the colorations color patches on the left, or from the channels separations, and proceeding with the dragging.
Create Colors in Color Library
Pressing the button (page+) under a library, a dialog opens with options to create new colors for the library such as new, copy, and duplicate. The new colors can be duplicated and renamed, copied, and pasted in other color libraries.
Newly created colors will contain color names in a sequence of C-1, C-2, etc.