We haven't considered adding a way for people to own games on itch.io that they already own on Steam. I need to think about it some more but it could be an interesting feature.
You can import a list of email addresses and generate keys from them using the Kickstarter import tool. We support a plain CSV format that includes a list of emails (maybe the tool should be renamed at some point). Once you've imported them you can send out a email to everyone using the email tool. More info here: https://itch.io/updates/improved-buyer-emailing-kickstarterintegration