In Magento 1.X you needed to expand export/import using special extensions. In Magento 2.0 there is a possibility to import custom options from the box.
To start, we can create a custom option for the product and save the product.
After that we can import the custom option created for any other product.
Select any product and enter the Custom options section, then select import custom options.
You will see the list of products from which you can import custom options.
Select the necessary product and import it.