Like the title say, is it possible? When I release my game (I wish) even on Steam, I want to put the same price here and there, But I would like to make sure that those who donated to me here since that day also have the Steam version to thank them for their support. Is there a simple and safe way to understand who paid?

You can give keys for other services like Steam to people who buy your game on itch.io, but I'm not sure how it works for people who already did. See the Download Keys FAQ.