How to put an already installed game into steam

[Asshat ツ]

So, I downloaded and installed RIFT™. That happened from the official website. Later on, I realized that RIFT™ is also available on Steam. Soo, now I would like to play it through steam but without installing it again. Because when I go to the game's page, it saya that I already have it etc. and then Play Now but instead of launching the game, it leads me to the installation stuff, deciding in which folder I want to install it etc.

So, tl;dr I already have RIFT but I downloaded it from the official website. How to add it so that steam detects that I'm playing it?

[Debaser]

If you want the full functionality and steam to track your stats you will need to install it via steam as steam installs its own files as well. You could add it as a non steam game and get the overlay but no stats will be tracked.

[Hextravert]

Start downloading/installing the Steam version—then pause it.

Now go to the folder where your ''official'' files are located and copy those. Paste them in the Steam folder.

Resume download and you will be playing in no time.

[spoof]

actually put the files in that folder first and steam will just check what files you have when installing.