I bought the game when it was released for Windows 8 but I no longer have Windows 8 and can;t transfer it to Steam. Is there another way to transfer it?
Log into the Windows 8 Store online, I suppose 
The only way for you to claim a steam key is through the windows 8 game itself. Sorry, but we simply don’t have any other way to confirm that you actually own the game.
Cant you get the users account and e-mail, and ask Microsoft to handle it?
no, we investigated other possibilities. no chance.
Well, Microsoft is weird.