Hey, I just wanted to send through this in case it might help you to figure this out. Looks like you just have to generate bulk keys on Steam and enter them into the "external download keys" page as shown here. Then itch automatically gives them out to users who click the "Get Steam Key" button on the game's page.
At the bottom there it says "If your game has been for sale on itch.io for a while before being on Steam those who have purchased it can go back to their download page and still get a key."
Good luck!