How can I add a pattern to existing color library in Illustrator?

Draw a Shape and duplicate it to create the pattern you want.

Group (CTRL,CMD+G) the objects.

Drag the group into the swatch panel.

Apply! Voila!

Bonus: Scale (CTRL, CMD + S) scale the pattern alone by unchecking the object box.

Draw a Shape and duplicate it to create the pattern you want

Group (CTRL,CMD+G) the objects

Drag the group into the swatch panel

Creat the patteren and drag + drop into swatches menu.

selection pattern and drag into color plate

Draw ur shapes, group it and drag it to the swatch panel 

Through object menu > pattern > make.

Or simply drag your object to swatches panel then apply it to any future dawing.

If you want to include pattern in your liberary R-click then choose add to liberary.

If you want to include pattern colors in your liberary select your pattern then from swatches panel choose add selected colors then choose these swatches & from swatches bottom icons choos add selected swatches& color groups to xurrent liberary.

Select the Object and Drag Pattern by Swatches

Make a pattern or new color then drag it to the color swatches.

Creat the patteren  as you like and make it group (CTRL,CMD+G)and drag in to swatch 

YOU CAN CREATE A NEW LAYER FIRST  CTRL+N THEN FILL YOUR COLOUR AND USE YOUR DIALOGUE BOX TO MANIPULATE .

you want to add created element in the swatch panel. just selecte that element and drag in the swatch panel.