To add additional sizes or colors to a product that is already in your catalog, you should use the Show POS button at the bottom of the Edit Option page of the existing product.
Go to that product's edit option page and click on the Show POS button.
You should see some results .
Check off the box of the item you want to add and select Add.
The items should be added to the list above. Then hit Save.
If you do not get a result after clicking Show POS, try using another search term such as the Style# or part of the item name.