Please, please, please, don't inflict that on yourself. Steam utterly and completely sucks for games like this, because, if the Steam version isn't completely bang up to date, it wars with the version downloaded by the official launcher, causing unnecessary client wiping and downloading galore.
However, if you are some kind of TRIBBLE and want to do this anyway, you should simply be able to log into Steam, find it in the store, then download it, and this will add it into your games list.
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However, if you are some kind of TRIBBLE and want to do this anyway, you should simply be able to log into Steam, find it in the store, then download it, and this will add it into your games list.