Err... you have to figure that out on the Steam side?
Ouch! They don't have any system to let us know who purchased the game, as far I know… -_-'
I don't know anything about selling on steam but I bet you could put a menu option in your game that ties in with the Steam API and requests an itch key from your own server.
That's a good idea, but I removed my games from Steam, I don't know if their API still works for them.
Also, I have a RPG Maker 2003 game that doesn't have support to the Steam API anyway. =/