Add Cities skylines to steam?

[LeeRaywen]

Can I add Cities Skylines to my Steam games if I have the disc-/boxed version of the game. So can I use the Steam-workshop for mods with the disc version?
(I'm thinking of buying it on amazon)
VG LeeRaywen

 

[AndrewT]

The boxed version is already a Steam version - the game is not available in a non-Steam version in fact.