You can use itch.io's Kickstarter import tool to import all of your backers directly from Kickstarter. It's completely free! Instead of copying backer information around you can upload the CSV file generated directly by Kickstarter. Each backer you import will get their own download URL on itch.io to access any files or keys you provide.
When doing the import ensure that "Unlock external keys" so any additional keys you've added, like Steam keys, will be available to the backers.
Next, you'll need to upload your Steam keys. From your game's dashboard head to the Distribute tab and select External keys on the sidebar. Provide any keys you have from this screen. In addition to anyone who buys your game on itch.io, the backers you've imported will now have access to the keys as well.
All that's left is to notify the backers that the keys have arrived, and that they can access them from your itch.io download URL. Head to the Interact tab on your game's edit page, and create a new email. You can select the backer groups you generated as the recipient of the email. Ensure to select Include download link at bottom of email so that each email includes the unique download URL that will grant the backers access to the game.
If you have any questions or issues post below.