Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - Latest News Announcing Divinity: Original Sin Enhanced Edition

There is no retail release for the PC version of the Enhanced Edition.
Redeeming a Steam key for the retail version of D:OS would give you the EE, as well (since the key would have been created before the EE was released). It would be a separate entry in your Steam library, so would be the full download whether you bought the retail version of D:OS or bought the EE directly from Steam or GOG.